Merge "Skip UniqueConstraint marked by unique=True in tometadata"

3 files changed, 43 insertions, 1 deletions

diff --git a/doc/build/changelog/changelog_10.rst b/doc/build/changelog/changelog_10.rst
index 5f9521c47..b59ab392e 100644
--- a/doc/build/changelog/changelog_10.rst
+++ b/doc/build/changelog/changelog_10.rst
@@ -19,6 +19,14 @@
     :version: 1.0.14
 
     .. change::
+        :tags: bug, sql
+        :tickets: 3721
+
+        Fixed bug whereby :meth:`.Table.tometadata` would make a duplicate
+        :class:`.UniqueConstraint` for each :class:`.Column` object that
+        featured the ``unique=True`` parameter.
+
+    .. change::
         :tags: bug, engine, postgresql
         :tickets: 3716
 
diff --git a/lib/sqlalchemy/sql/schema.py b/lib/sqlalchemy/sql/schema.py
index 64692644c..cb01a49e3 100644
--- a/lib/sqlalchemy/sql/schema.py
+++ b/lib/sqlalchemy/sql/schema.py
@@ -852,8 +852,14 @@ class Table(DialectKWArgs, SchemaItem, TableClause):
                         schema if referred_schema == self.schema else None)
                 table.append_constraint(
                     c.copy(schema=fk_constraint_schema, target_table=table))
-
             elif not c._type_bound:
+                # skip unique constraints that would be generated
+                # by the 'unique' flag on Column
+                if isinstance(c, UniqueConstraint) and \
+                    len(c.columns) == 1 and \
+                        list(c.columns)[0].unique:
+                    continue
+
                 table.append_constraint(
                     c.copy(schema=schema, target_table=table))
         for index in self.indexes:
diff --git a/test/sql/test_metadata.py b/test/sql/test_metadata.py
index 449956fcd..344cfefa5 100644
--- a/test/sql/test_metadata.py
+++ b/test/sql/test_metadata.py
@@ -1113,6 +1113,34 @@ class ToMetaDataTest(fixtures.TestBase, ComparesTables):
         eq_(str(table_c.join(table2_c).onclause),
             'mytable.myid = othertable.myid')
 
+    def test_unique_true_flag(self):
+        meta = MetaData()
+
+        table = Table('mytable', meta, Column('x', Integer, unique=True))
+
+        m2 = MetaData()
+
+        t2 = table.tometadata(m2)
+
+        eq_(
+            len([
+                const for const
+                in t2.constraints
+                if isinstance(const, UniqueConstraint)]),
+            1
+        )
+
+    def test_index_true_flag(self):
+        meta = MetaData()
+
+        table = Table('mytable', meta, Column('x', Integer, index=True))
+
+        m2 = MetaData()
+
+        t2 = table.tometadata(m2)
+
+        eq_(len(t2.indexes), 1)
+
 
 class InfoTest(fixtures.TestBase):
     def test_metadata_info(self):