Add cookbook recipe for don't render DROP INDEX

Filters out DropIndexOp when there is a corresponding
DropTableOp.

Change-Id: I7baadf6e5b9f669c875aeeaccefb19cb5e105953
Fixes: #467

diff --git a/docs/build/cookbook.rst b/docs/build/cookbook.rst
index 26932e02f72338eaff5404a2ef50c2b57deb4e27..c4a4eef65f395660f11d4da10a8a3e7b53eed124 100644 (file)
--- a/docs/build/cookbook.rst
+++ b/docs/build/cookbook.rst
@@ -809,6 +809,98 @@ any operations::
             with context.begin_transaction():
                 context.run_migrations()
 
+Don't emit DROP INDEX when the table is to be dropped as well
+=============================================================
+
+MySQL may complain when dropping an index that is against a column
+that also has a foreign key constraint on it.   If the table is to be dropped
+in any case, the DROP INDEX isn't necessary.  This recipe will process the set
+of autogenerate directives such that all :class:`.DropIndexOp` directives
+are removed against tables that themselves are to be dropped::
+
+    def run_migrations_online():
+
+        # ...
+
+        from alembic.operations import ops
+
+        def process_revision_directives(context, revision, directives):
+            script = directives[0]
+
+            # process both "def upgrade()", "def downgrade()"
+            for directive in (script.upgrade_ops, script.downgrade_ops):
+
+                # make a set of tables that are being dropped within
+                # the migration function
+                tables_dropped = set()
+                for op in directive.ops:
+                    if isinstance(op, ops.DropTableOp):
+                        tables_dropped.add((op.table_name, op.schema))
+
+                # now rewrite the list of "ops" such that DropIndexOp
+                # is removed for those tables.   Needs a recursive function.
+                directive.ops = list(
+                    _filter_drop_indexes(directive.ops, tables_dropped)
+                )
+
+        def _filter_drop_indexes(directives, tables_dropped):
+            # given a set of (tablename, schemaname) to be dropped, filter
+            # out DropIndexOp from the list of directives and yield the result.
+
+            for directive in directives:
+                # ModifyTableOps is a container of ALTER TABLE types of
+                # commands.  process those in place recursively.
+                if isinstance(directive, ops.ModifyTableOps) and \
+                        (directive.table_name, directive.schema) in tables_dropped:
+                    directive.ops = list(
+                        _filter_drop_indexes(directive.ops, tables_dropped)
+                    )
+
+                    # if we emptied out the directives, then skip the
+                    # container altogether.
+                    if not directive.ops:
+                        continue
+                elif isinstance(directive, ops.DropIndexOp) and \
+                        (directive.table_name, directive.schema) in tables_dropped:
+                    # we found a target DropIndexOp.   keep looping
+                    continue
+
+                # otherwise if not filtered, yield out the directive
+                yield directive
+
+        # connectable = ...
+
+        with connectable.connect() as connection:
+            context.configure(
+                connection=connection,
+                target_metadata=target_metadata,
+                process_revision_directives=process_revision_directives
+            )
+
+            with context.begin_transaction():
+                context.run_migrations()
+
+
+Whereas autogenerate, when dropping two tables with a foreign key and
+an index, would previously generate something like::
+
+    def downgrade():
+        # ### commands auto generated by Alembic - please adjust! ###
+        op.drop_index(op.f('ix_b_aid'), table_name='b')
+        op.drop_table('b')
+        op.drop_table('a')
+        # ### end Alembic commands ###
+
+With the above rewriter, it generates as::
+
+    def downgrade():
+        # ### commands auto generated by Alembic - please adjust! ###
+        op.drop_table('b')
+        op.drop_table('a')
+        # ### end Alembic commands ###
+
+
+
 Don't emit CREATE TABLE statements for Views
 ============================================