Skip UniqueConstraint marked by unique=True in tometadata

Fixes an issue where a Column would be copied with unique=True
and at the same time the UniqueConstraint would also be copied,
leading to duplicate UniqueConstraints in the target table,
when tometadata() is used.   Imitates the same logic used
by index=True/Index to avoid duplicates.  For some reason
a fix was implemented for Index long ago but never for
UniqueConstraint.

Change-Id: Ie622ee912a6fb8bf0ea900a8b09d78c7ebc79fc0
Fixes: #3721

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):