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//lib/python2.7/site-packages/alembic/autogenerate/compare.py

import contextlib
import logging
import re

from sqlalchemy import event
from sqlalchemy import schema as sa_schema
from sqlalchemy import types as sqltypes
from sqlalchemy.engine.reflection import Inspector
from sqlalchemy.util import OrderedSet

from alembic.ddl.base import _fk_spec
from .render import _user_defined_render
from .. import util
from ..operations import ops
from ..util import compat
from ..util import sqla_compat

log = logging.getLogger(__name__)

def _populate_migration_script(autogen_context, migration_script):
    upgrade_ops = migration_script.upgrade_ops_list[-1]
    downgrade_ops = migration_script.downgrade_ops_list[-1]

_produce_net_changes(autogen_context, upgrade_ops)
    upgrade_ops.reverse_into(downgrade_ops)

comparators = util.Dispatcher(uselist=True)

def _produce_net_changes(autogen_context, upgrade_ops):

connection = autogen_context.connection
    include_schemas = autogen_context.opts.get("include_schemas", False)

inspector = Inspector.from_engine(connection)

default_schema = connection.dialect.default_schema_name
    if include_schemas:
        schemas = set(inspector.get_schema_names())
        # replace default schema name with None
        schemas.discard("information_schema")
        # replace the "default" schema with None
        schemas.discard(default_schema)
        schemas.add(None)
    else:
        schemas = [None]

comparators.dispatch("schema", autogen_context.dialect.name)(
        autogen_context, upgrade_ops, schemas
    )

@comparators.dispatch_for("schema")
def _autogen_for_tables(autogen_context, upgrade_ops, schemas):
    inspector = autogen_context.inspector

conn_table_names = set()

version_table_schema = (
        autogen_context.migration_context.version_table_schema
    )
    version_table = autogen_context.migration_context.version_table

for s in schemas:
        tables = set(inspector.get_table_names(schema=s))
        if s == version_table_schema:
            tables = tables.difference(
                [autogen_context.migration_context.version_table]
            )
        conn_table_names.update(zip([s] * len(tables), tables))

metadata_table_names = OrderedSet(
        [(table.schema, table.name) for table in autogen_context.sorted_tables]
    ).difference([(version_table_schema, version_table)])

_compare_tables(
        conn_table_names,
        metadata_table_names,
        inspector,
        upgrade_ops,
        autogen_context,
    )

def _compare_tables(
    conn_table_names,
    metadata_table_names,
    inspector,
    upgrade_ops,
    autogen_context,
):

default_schema = inspector.bind.dialect.default_schema_name

# tables coming from the connection will not have "schema"
    # set if it matches default_schema_name; so we need a list
    # of table names from local metadata that also have "None" if schema
    # == default_schema_name.  Most setups will be like this anyway but
    # some are not (see #170)
    metadata_table_names_no_dflt_schema = OrderedSet(
        [
            (schema if schema != default_schema else None, tname)
            for schema, tname in metadata_table_names
        ]
    )

# to adjust for the MetaData collection storing the tables either
    # as "schemaname.tablename" or just "tablename", create a new lookup
    # which will match the "non-default-schema" keys to the Table object.
    tname_to_table = dict(
        (
            no_dflt_schema,
            autogen_context.table_key_to_table[
                sa_schema._get_table_key(tname, schema)
            ],
        )
        for no_dflt_schema, (schema, tname) in zip(
            metadata_table_names_no_dflt_schema, metadata_table_names
        )
    )
    metadata_table_names = metadata_table_names_no_dflt_schema

for s, tname in metadata_table_names.difference(conn_table_names):
        name = "%s.%s" % (s, tname) if s else tname
        metadata_table = tname_to_table[(s, tname)]
        if autogen_context.run_filters(
            metadata_table, tname, "table", False, None
        ):
            upgrade_ops.ops.append(
                ops.CreateTableOp.from_table(metadata_table)
            )
            log.info("Detected added table %r", name)
            modify_table_ops = ops.ModifyTableOps(tname, [], schema=s)

comparators.dispatch("table")(
                autogen_context,
                modify_table_ops,
                s,
                tname,
                None,
                metadata_table,
            )
            if not modify_table_ops.is_empty():
                upgrade_ops.ops.append(modify_table_ops)

removal_metadata = sa_schema.MetaData()
    for s, tname in conn_table_names.difference(metadata_table_names):
        name = sa_schema._get_table_key(tname, s)
        exists = name in removal_metadata.tables
        t = sa_schema.Table(tname, removal_metadata, schema=s)

if not exists:
            event.listen(
                t,
                "column_reflect",
                # fmt: off
                autogen_context.migration_context.impl.
                _compat_autogen_column_reflect
                (inspector),
                # fmt: on
            )
            inspector.reflecttable(t, None)
        if autogen_context.run_filters(t, tname, "table", True, None):

modify_table_ops = ops.ModifyTableOps(tname, [], schema=s)

comparators.dispatch("table")(
                autogen_context, modify_table_ops, s, tname, t, None
            )
            if not modify_table_ops.is_empty():
                upgrade_ops.ops.append(modify_table_ops)

upgrade_ops.ops.append(ops.DropTableOp.from_table(t))
            log.info("Detected removed table %r", name)

existing_tables = conn_table_names.intersection(metadata_table_names)

existing_metadata = sa_schema.MetaData()
    conn_column_info = {}
    for s, tname in existing_tables:
        name = sa_schema._get_table_key(tname, s)
        exists = name in existing_metadata.tables
        t = sa_schema.Table(tname, existing_metadata, schema=s)
        if not exists:
            event.listen(
                t,
                "column_reflect",
                # fmt: off
                autogen_context.migration_context.impl.
                _compat_autogen_column_reflect(inspector),
                # fmt: on
            )
            inspector.reflecttable(t, None)
        conn_column_info[(s, tname)] = t

for s, tname in sorted(existing_tables, key=lambda x: (x[0] or "", x[1])):
        s = s or None
        name = "%s.%s" % (s, tname) if s else tname
        metadata_table = tname_to_table[(s, tname)]
        conn_table = existing_metadata.tables[name]

if autogen_context.run_filters(
            metadata_table, tname, "table", False, conn_table
        ):

modify_table_ops = ops.ModifyTableOps(tname, [], schema=s)
            with _compare_columns(
                s,
                tname,
                conn_table,
                metadata_table,
                modify_table_ops,
                autogen_context,
                inspector,
            ):

comparators.dispatch("table")(
                    autogen_context,
                    modify_table_ops,
                    s,
                    tname,
                    conn_table,
                    metadata_table,
                )

if not modify_table_ops.is_empty():
                upgrade_ops.ops.append(modify_table_ops)

def _make_index(params, conn_table):
    ix = sa_schema.Index(
        params["name"],
        *[conn_table.c[cname] for cname in params["column_names"]],
        unique=params["unique"]
    )
    if "duplicates_constraint" in params:
        ix.info["duplicates_constraint"] = params["duplicates_constraint"]
    return ix

def _make_unique_constraint(params, conn_table):
    uq = sa_schema.UniqueConstraint(
        *[conn_table.c[cname] for cname in params["column_names"]],
        name=params["name"]
    )
    if "duplicates_index" in params:
        uq.info["duplicates_index"] = params["duplicates_index"]

return uq

def _make_foreign_key(params, conn_table):
    tname = params["referred_table"]
    if params["referred_schema"]:
        tname = "%s.%s" % (params["referred_schema"], tname)

options = params.get("options", {})

const = sa_schema.ForeignKeyConstraint(
        [conn_table.c[cname] for cname in params["constrained_columns"]],
        ["%s.%s" % (tname, n) for n in params["referred_columns"]],
        onupdate=options.get("onupdate"),
        ondelete=options.get("ondelete"),
        deferrable=options.get("deferrable"),
        initially=options.get("initially"),
        name=params["name"],
    )
    # needed by 0.7
    conn_table.append_constraint(const)
    return const

@contextlib.contextmanager
def _compare_columns(
    schema,
    tname,
    conn_table,
    metadata_table,
    modify_table_ops,
    autogen_context,
    inspector,
):
    name = "%s.%s" % (schema, tname) if schema else tname
    metadata_cols_by_name = dict(
        (c.name, c) for c in metadata_table.c if not c.system
    )
    conn_col_names = dict((c.name, c) for c in conn_table.c)
    metadata_col_names = OrderedSet(sorted(metadata_cols_by_name))

for cname in metadata_col_names.difference(conn_col_names):
        if autogen_context.run_filters(
            metadata_cols_by_name[cname], cname, "column", False, None
        ):
            modify_table_ops.ops.append(
                ops.AddColumnOp.from_column_and_tablename(
                    schema, tname, metadata_cols_by_name[cname]
                )
            )
            log.info("Detected added column '%s.%s'", name, cname)

for colname in metadata_col_names.intersection(conn_col_names):
        metadata_col = metadata_cols_by_name[colname]
        conn_col = conn_table.c[colname]
        if not autogen_context.run_filters(
            metadata_col, colname, "column", False, conn_col
        ):
            continue
        alter_column_op = ops.AlterColumnOp(tname, colname, schema=schema)

comparators.dispatch("column")(
            autogen_context,
            alter_column_op,
            schema,
            tname,
            colname,
            conn_col,
            metadata_col,
        )

if alter_column_op.has_changes():
            modify_table_ops.ops.append(alter_column_op)

yield

for cname in set(conn_col_names).difference(metadata_col_names):
        if autogen_context.run_filters(
            conn_table.c[cname], cname, "column", True, None
        ):
            modify_table_ops.ops.append(
                ops.DropColumnOp.from_column_and_tablename(
                    schema, tname, conn_table.c[cname]
                )
            )
            log.info("Detected removed column '%s.%s'", name, cname)

class _constraint_sig(object):
    def md_name_to_sql_name(self, context):
        return sqla_compat._get_constraint_final_name(
            self.const, context.dialect
        )

def __eq__(self, other):
        return self.const == other.const

def __ne__(self, other):
        return self.const != other.const

def __hash__(self):
        return hash(self.const)

class _uq_constraint_sig(_constraint_sig):
    is_index = False
    is_unique = True

def __init__(self, const):
        self.const = const
        self.name = const.name
        self.sig = tuple(sorted([col.name for col in const.columns]))

@property
    def column_names(self):
        return [col.name for col in self.const.columns]

class _ix_constraint_sig(_constraint_sig):
    is_index = True

def __init__(self, const):
        self.const = const
        self.name = const.name
        self.sig = tuple(sorted([col.name for col in const.columns]))
        self.is_unique = bool(const.unique)

def md_name_to_sql_name(self, context):
        return sqla_compat._get_index_final_name(context.dialect, self.const)

@property
    def column_names(self):
        return sqla_compat._get_index_column_names(self.const)

class _fk_constraint_sig(_constraint_sig):
    def __init__(self, const, include_options=False):
        self.const = const
        self.name = const.name

(
            self.source_schema,
            self.source_table,
            self.source_columns,
            self.target_schema,
            self.target_table,
            self.target_columns,
            onupdate,
            ondelete,
            deferrable,
            initially,
        ) = _fk_spec(const)

self.sig = (
            self.source_schema,
            self.source_table,
            tuple(self.source_columns),
            self.target_schema,
            self.target_table,
            tuple(self.target_columns),
        )
        if include_options:
            self.sig += (
                (None if onupdate.lower() == "no action" else onupdate.lower())
                if onupdate
                else None,
                (None if ondelete.lower() == "no action" else ondelete.lower())
                if ondelete
                else None,
                # convert initially + deferrable into one three-state value
                "initially_deferrable"
                if initially and initially.lower() == "deferred"
                else "deferrable"
                if deferrable
                else "not deferrable",
            )

@comparators.dispatch_for("table")
def _compare_indexes_and_uniques(
    autogen_context, modify_ops, schema, tname, conn_table, metadata_table
):

inspector = autogen_context.inspector
    is_create_table = conn_table is None
    is_drop_table = metadata_table is None

# 1a. get raw indexes and unique constraints from metadata ...
    if metadata_table is not None:
        metadata_unique_constraints = set(
            uq
            for uq in metadata_table.constraints
            if isinstance(uq, sa_schema.UniqueConstraint)
        )
        metadata_indexes = set(metadata_table.indexes)
    else:
        metadata_unique_constraints = set()
        metadata_indexes = set()

conn_uniques = conn_indexes = frozenset()

supports_unique_constraints = False

unique_constraints_duplicate_unique_indexes = False

if conn_table is not None:
        # 1b. ... and from connection, if the table exists
        if hasattr(inspector, "get_unique_constraints"):
            try:
                conn_uniques = inspector.get_unique_constraints(
                    tname, schema=schema
                )
                supports_unique_constraints = True
            except NotImplementedError:
                pass
            except TypeError:
                # number of arguments is off for the base
                # method in SQLAlchemy due to the cache decorator
                # not being present
                pass
            else:
                for uq in conn_uniques:
                    if uq.get("duplicates_index"):
                        unique_constraints_duplicate_unique_indexes = True
        try:
            conn_indexes = inspector.get_indexes(tname, schema=schema)
        except NotImplementedError:
            pass

# 2. convert conn-level objects from raw inspector records
        # into schema objects
        if is_drop_table:
            # for DROP TABLE uniques are inline, don't need them
            conn_uniques = set()
        else:
            conn_uniques = set(
                _make_unique_constraint(uq_def, conn_table)
                for uq_def in conn_uniques
            )

conn_indexes = set(_make_index(ix, conn_table) for ix in conn_indexes)

# 2a. if the dialect dupes unique indexes as unique constraints
    # (mysql and oracle), correct for that

if unique_constraints_duplicate_unique_indexes:
        _correct_for_uq_duplicates_uix(
            conn_uniques,
            conn_indexes,
            metadata_unique_constraints,
            metadata_indexes,
        )

# 3. give the dialect a chance to omit indexes and constraints that
    # we know are either added implicitly by the DB or that the DB
    # can't accurately report on
    autogen_context.migration_context.impl.correct_for_autogen_constraints(
        conn_uniques,
        conn_indexes,
        metadata_unique_constraints,
        metadata_indexes,
    )

# 4. organize the constraints into "signature" collections, the
    # _constraint_sig() objects provide a consistent facade over both
    # Index and UniqueConstraint so we can easily work with them
    # interchangeably
    metadata_unique_constraints = set(
        _uq_constraint_sig(uq) for uq in metadata_unique_constraints
    )

metadata_indexes = set(_ix_constraint_sig(ix) for ix in metadata_indexes)

conn_unique_constraints = set(
        _uq_constraint_sig(uq) for uq in conn_uniques
    )

conn_indexes = set(_ix_constraint_sig(ix) for ix in conn_indexes)

# 5. index things by name, for those objects that have names
    metadata_names = dict(
        (c.md_name_to_sql_name(autogen_context), c)
        for c in metadata_unique_constraints.union(metadata_indexes)
        if isinstance(c, _ix_constraint_sig)
        or sqla_compat._constraint_is_named(c.const, autogen_context.dialect)
    )

conn_uniques_by_name = dict((c.name, c) for c in conn_unique_constraints)
    conn_indexes_by_name = dict((c.name, c) for c in conn_indexes)

conn_names = dict(
        (c.name, c)
        for c in conn_unique_constraints.union(conn_indexes)
        if c.name is not None
    )

doubled_constraints = dict(
        (name, (conn_uniques_by_name[name], conn_indexes_by_name[name]))
        for name in set(conn_uniques_by_name).intersection(
            conn_indexes_by_name
        )
    )

# 6. index things by "column signature", to help with unnamed unique
    # constraints.
    conn_uniques_by_sig = dict((uq.sig, uq) for uq in conn_unique_constraints)
    metadata_uniques_by_sig = dict(
        (uq.sig, uq) for uq in metadata_unique_constraints
    )
    metadata_indexes_by_sig = dict((ix.sig, ix) for ix in metadata_indexes)
    unnamed_metadata_uniques = dict(
        (uq.sig, uq)
        for uq in metadata_unique_constraints
        if not sqla_compat._constraint_is_named(
            uq.const, autogen_context.dialect
        )
    )

# assumptions:
    # 1. a unique constraint or an index from the connection *always*
    #    has a name.
    # 2. an index on the metadata side *always* has a name.
    # 3. a unique constraint on the metadata side *might* have a name.
    # 4. The backend may double up indexes as unique constraints and
    #    vice versa (e.g. MySQL, Postgresql)

def obj_added(obj):
        if obj.is_index:
            if autogen_context.run_filters(
                obj.const, obj.name, "index", False, None
            ):
                modify_ops.ops.append(ops.CreateIndexOp.from_index(obj.const))
                log.info(
                    "Detected added index '%s' on %s",
                    obj.name,
                    ", ".join(["'%s'" % obj.column_names]),
                )
        else:
            if not supports_unique_constraints:
                # can't report unique indexes as added if we don't
                # detect them
                return
            if is_create_table or is_drop_table:
                # unique constraints are created inline with table defs
                return
            if autogen_context.run_filters(
                obj.const, obj.name, "unique_constraint", False, None
            ):
                modify_ops.ops.append(
                    ops.AddConstraintOp.from_constraint(obj.const)
                )
                log.info(
                    "Detected added unique constraint '%s' on %s",
                    obj.name,
                    ", ".join(["'%s'" % obj.column_names]),
                )

def obj_removed(obj):
        if obj.is_index:
            if obj.is_unique and not supports_unique_constraints:
                # many databases double up unique constraints
                # as unique indexes.  without that list we can't
                # be sure what we're doing here
                return

if autogen_context.run_filters(
                obj.const, obj.name, "index", True, None
            ):
                modify_ops.ops.append(ops.DropIndexOp.from_index(obj.const))
                log.info(
                    "Detected removed index '%s' on '%s'", obj.name, tname
                )
        else:
            if is_create_table or is_drop_table:
                # if the whole table is being dropped, we don't need to
                # consider unique constraint separately
                return
            if autogen_context.run_filters(
                obj.const, obj.name, "unique_constraint", True, None
            ):
                modify_ops.ops.append(
                    ops.DropConstraintOp.from_constraint(obj.const)
                )
                log.info(
                    "Detected removed unique constraint '%s' on '%s'",
                    obj.name,
                    tname,
                )

def obj_changed(old, new, msg):
        if old.is_index:
            if autogen_context.run_filters(
                new.const, new.name, "index", False, old.const
            ):
                log.info(
                    "Detected changed index '%s' on '%s':%s",
                    old.name,
                    tname,
                    ", ".join(msg),
                )
                modify_ops.ops.append(ops.DropIndexOp.from_index(old.const))
                modify_ops.ops.append(ops.CreateIndexOp.from_index(new.const))
        else:
            if autogen_context.run_filters(
                new.const, new.name, "unique_constraint", False, old.const
            ):
                log.info(
                    "Detected changed unique constraint '%s' on '%s':%s",
                    old.name,
                    tname,
                    ", ".join(msg),
                )
                modify_ops.ops.append(
                    ops.DropConstraintOp.from_constraint(old.const)
                )
                modify_ops.ops.append(
                    ops.AddConstraintOp.from_constraint(new.const)
                )

for added_name in sorted(set(metadata_names).difference(conn_names)):
        obj = metadata_names[added_name]
        obj_added(obj)

for existing_name in sorted(set(metadata_names).intersection(conn_names)):
        metadata_obj = metadata_names[existing_name]

if existing_name in doubled_constraints:
            conn_uq, conn_idx = doubled_constraints[existing_name]
            if metadata_obj.is_index:
                conn_obj = conn_idx
            else:
                conn_obj = conn_uq
        else:
            conn_obj = conn_names[existing_name]

if conn_obj.is_index != metadata_obj.is_index:
            obj_removed(conn_obj)
            obj_added(metadata_obj)
        else:
            msg = []
            if conn_obj.is_unique != metadata_obj.is_unique:
                msg.append(
                    " unique=%r to unique=%r"
                    % (conn_obj.is_unique, metadata_obj.is_unique)
                )
            if conn_obj.sig != metadata_obj.sig:
                msg.append(
                    " columns %r to %r" % (conn_obj.sig, metadata_obj.sig)
                )

if msg:
                obj_changed(conn_obj, metadata_obj, msg)

for removed_name in sorted(set(conn_names).difference(metadata_names)):
        conn_obj = conn_names[removed_name]
        if not conn_obj.is_index and conn_obj.sig in unnamed_metadata_uniques:
            continue
        elif removed_name in doubled_constraints:
            if (
                conn_obj.sig not in metadata_indexes_by_sig
                and conn_obj.sig not in metadata_uniques_by_sig
            ):
                conn_uq, conn_idx = doubled_constraints[removed_name]
                obj_removed(conn_uq)
                obj_removed(conn_idx)
        else:
            obj_removed(conn_obj)

for uq_sig in unnamed_metadata_uniques:
        if uq_sig not in conn_uniques_by_sig:
            obj_added(unnamed_metadata_uniques[uq_sig])

def _correct_for_uq_duplicates_uix(
    conn_unique_constraints,
    conn_indexes,
    metadata_unique_constraints,
    metadata_indexes,
):
    # dedupe unique indexes vs. constraints, since MySQL / Oracle
    # doesn't really have unique constraints as a separate construct.
    # but look in the metadata and try to maintain constructs
    # that already seem to be defined one way or the other
    # on that side.  This logic was formerly local to MySQL dialect,
    # generalized to Oracle and others. See #276
    metadata_uq_names = set(
        [
            cons.name
            for cons in metadata_unique_constraints
            if cons.name is not None
        ]
    )

unnamed_metadata_uqs = set(
        [
            _uq_constraint_sig(cons).sig
            for cons in metadata_unique_constraints
            if cons.name is None
        ]
    )

metadata_ix_names = set(
        [cons.name for cons in metadata_indexes if cons.unique]
    )
    conn_ix_names = dict(
        (cons.name, cons) for cons in conn_indexes if cons.unique
    )

uqs_dupe_indexes = dict(
        (cons.name, cons)
        for cons in conn_unique_constraints
        if cons.info["duplicates_index"]
    )

for overlap in uqs_dupe_indexes:
        if overlap not in metadata_uq_names:
            if (
                _uq_constraint_sig(uqs_dupe_indexes[overlap]).sig
                not in unnamed_metadata_uqs
            ):

conn_unique_constraints.discard(uqs_dupe_indexes[overlap])
        elif overlap not in metadata_ix_names:
            conn_indexes.discard(conn_ix_names[overlap])

@comparators.dispatch_for("column")
def _compare_nullable(
    autogen_context,
    alter_column_op,
    schema,
    tname,
    cname,
    conn_col,
    metadata_col,
):

# work around SQLAlchemy issue #3023
    if metadata_col.primary_key:
        return

metadata_col_nullable = metadata_col.nullable
    conn_col_nullable = conn_col.nullable
    alter_column_op.existing_nullable = conn_col_nullable

if conn_col_nullable is not metadata_col_nullable:
        alter_column_op.modify_nullable = metadata_col_nullable
        log.info(
            "Detected %s on column '%s.%s'",
            "NULL" if metadata_col_nullable else "NOT NULL",
            tname,
            cname,
        )

@comparators.dispatch_for("column")
def _setup_autoincrement(
    autogen_context,
    alter_column_op,
    schema,
    tname,
    cname,
    conn_col,
    metadata_col,
):

if metadata_col.table._autoincrement_column is metadata_col:
        alter_column_op.kw["autoincrement"] = True
    elif metadata_col.autoincrement is True:
        alter_column_op.kw["autoincrement"] = True
    elif metadata_col.autoincrement is False:
        alter_column_op.kw["autoincrement"] = False

@comparators.dispatch_for("column")
def _compare_type(
    autogen_context,
    alter_column_op,
    schema,
    tname,
    cname,
    conn_col,
    metadata_col,
):

conn_type = conn_col.type
    alter_column_op.existing_type = conn_type
    metadata_type = metadata_col.type
    if conn_type._type_affinity is sqltypes.NullType:
        log.info(
            "Couldn't determine database type " "for column '%s.%s'",
            tname,
            cname,
        )
        return
    if metadata_type._type_affinity is sqltypes.NullType:
        log.info(
            "Column '%s.%s' has no type within " "the model; can't compare",
            tname,
            cname,
        )
        return

isdiff = autogen_context.migration_context._compare_type(
        conn_col, metadata_col
    )

if isdiff:
        alter_column_op.modify_type = metadata_type
        log.info(
            "Detected type change from %r to %r on '%s.%s'",
            conn_type,
            metadata_type,
            tname,
            cname,
        )

def _render_server_default_for_compare(
    metadata_default, metadata_col, autogen_context
):
    rendered = _user_defined_render(
        "server_default", metadata_default, autogen_context
    )
    if rendered is not False:
        return rendered

if isinstance(metadata_default, sa_schema.DefaultClause):
        if isinstance(metadata_default.arg, compat.string_types):
            metadata_default = metadata_default.arg
        else:
            metadata_default = str(
                metadata_default.arg.compile(
                    dialect=autogen_context.dialect,
                    compile_kwargs={"literal_binds": True},
                )
            )
    if isinstance(metadata_default, compat.string_types):
        if metadata_col.type._type_affinity is sqltypes.String:
            metadata_default = re.sub(r"^'|'$", "", metadata_default)
            return repr(metadata_default)
        else:
            return metadata_default
    else:
        return None

@comparators.dispatch_for("column")
def _compare_server_default(
    autogen_context,
    alter_column_op,
    schema,
    tname,
    cname,
    conn_col,
    metadata_col,
):

metadata_default = metadata_col.server_default
    conn_col_default = conn_col.server_default
    if conn_col_default is None and metadata_default is None:
        return False

if sqla_compat.has_computed and isinstance(
        metadata_default, sa_schema.Computed
    ):
        # return False in case of a computed column as the server
        # default. Note that DDL for adding or removing "GENERATED AS" from
        # an existing column is not currently known for any backend.
        # Once SQLAlchemy can reflect "GENERATED" as the "computed" element,
        # we would also want to ignore and/or warn for changes vs. the
        # metadata (or support backend specific DDL if applicable).
        return False

rendered_metadata_default = _render_server_default_for_compare(
        metadata_default, metadata_col, autogen_context
    )

rendered_conn_default = (
        conn_col.server_default.arg.text if conn_col.server_default else None
    )

alter_column_op.existing_server_default = conn_col_default

isdiff = autogen_context.migration_context._compare_server_default(
        conn_col,
        metadata_col,
        rendered_metadata_default,
        rendered_conn_default,
    )
    if isdiff:
        alter_column_op.modify_server_default = metadata_default
        log.info("Detected server default on column '%s.%s'", tname, cname)

@comparators.dispatch_for("column")
def _compare_column_comment(
    autogen_context,
    alter_column_op,
    schema,
    tname,
    cname,
    conn_col,
    metadata_col,
):

if not sqla_compat._dialect_supports_comments(autogen_context.dialect):
        return

metadata_comment = metadata_col.comment
    conn_col_comment = conn_col.comment
    if conn_col_comment is None and metadata_comment is None:
        return False

alter_column_op.existing_comment = conn_col_comment

if conn_col_comment != metadata_comment:
        alter_column_op.modify_comment = metadata_comment
        log.info("Detected column comment '%s.%s'", tname, cname)

@comparators.dispatch_for("table")
def _compare_foreign_keys(
    autogen_context,
    modify_table_ops,
    schema,
    tname,
    conn_table,
    metadata_table,
):

# if we're doing CREATE TABLE, all FKs are created
    # inline within the table def
    if conn_table is None or metadata_table is None:
        return

inspector = autogen_context.inspector
    metadata_fks = set(
        fk
        for fk in metadata_table.constraints
        if isinstance(fk, sa_schema.ForeignKeyConstraint)
    )

conn_fks = inspector.get_foreign_keys(tname, schema=schema)

backend_reflects_fk_options = conn_fks and "options" in conn_fks[0]

conn_fks = set(_make_foreign_key(const, conn_table) for const in conn_fks)

# give the dialect a chance to correct the FKs to match more
    # closely
    autogen_context.migration_context.impl.correct_for_autogen_foreignkeys(
        conn_fks, metadata_fks
    )

metadata_fks = set(
        _fk_constraint_sig(fk, include_options=backend_reflects_fk_options)
        for fk in metadata_fks
    )

conn_fks = set(
        _fk_constraint_sig(fk, include_options=backend_reflects_fk_options)
        for fk in conn_fks
    )

conn_fks_by_sig = dict((c.sig, c) for c in conn_fks)
    metadata_fks_by_sig = dict((c.sig, c) for c in metadata_fks)

metadata_fks_by_name = dict(
        (c.name, c) for c in metadata_fks if c.name is not None
    )
    conn_fks_by_name = dict(
        (c.name, c) for c in conn_fks if c.name is not None
    )

def _add_fk(obj, compare_to):
        if autogen_context.run_filters(
            obj.const, obj.name, "foreign_key_constraint", False, compare_to
        ):
            modify_table_ops.ops.append(
                ops.CreateForeignKeyOp.from_constraint(const.const)
            )

log.info(
                "Detected added foreign key (%s)(%s) on table %s%s",
                ", ".join(obj.source_columns),
                ", ".join(obj.target_columns),
                "%s." % obj.source_schema if obj.source_schema else "",
                obj.source_table,
            )

def _remove_fk(obj, compare_to):
        if autogen_context.run_filters(
            obj.const, obj.name, "foreign_key_constraint", True, compare_to
        ):
            modify_table_ops.ops.append(
                ops.DropConstraintOp.from_constraint(obj.const)
            )
            log.info(
                "Detected removed foreign key (%s)(%s) on table %s%s",
                ", ".join(obj.source_columns),
                ", ".join(obj.target_columns),
                "%s." % obj.source_schema if obj.source_schema else "",
                obj.source_table,
            )

# so far it appears we don't need to do this by name at all.
    # SQLite doesn't preserve constraint names anyway

for removed_sig in set(conn_fks_by_sig).difference(metadata_fks_by_sig):
        const = conn_fks_by_sig[removed_sig]
        if removed_sig not in metadata_fks_by_sig:
            compare_to = (
                metadata_fks_by_name[const.name].const
                if const.name in metadata_fks_by_name
                else None
            )
            _remove_fk(const, compare_to)

for added_sig in set(metadata_fks_by_sig).difference(conn_fks_by_sig):
        const = metadata_fks_by_sig[added_sig]
        if added_sig not in conn_fks_by_sig:
            compare_to = (
                conn_fks_by_name[const.name].const
                if const.name in conn_fks_by_name
                else None
            )
            _add_fk(const, compare_to)

@comparators.dispatch_for("table")
def _compare_table_comment(
    autogen_context,
    modify_table_ops,
    schema,
    tname,
    conn_table,
    metadata_table,
):

if not sqla_compat._dialect_supports_comments(autogen_context.dialect):
        return

# if we're doing CREATE TABLE, comments will be created inline
    # with the create_table op.
    if conn_table is None or metadata_table is None:
        return

if conn_table.comment is None and metadata_table.comment is None:
        return

if metadata_table.comment is None and conn_table.comment is not None:
        modify_table_ops.ops.append(
            ops.DropTableCommentOp(
                tname, existing_comment=conn_table.comment, schema=schema
            )
        )
    elif metadata_table.comment != conn_table.comment:
        modify_table_ops.ops.append(
            ops.CreateTableCommentOp(
                tname,
                metadata_table.comment,
                existing_comment=conn_table.comment,
                schema=schema,
            )
        )