Workaround for passing model-classes as arguments.

Fixes #2131

peewee.py

@@ -1344,6 +1344,14 @@ def __sql__(self, ctx):
             # For multi-part values (e.g. lists of IDs).
             return ctx.sql(EnclosedNodeList(self.values))
 
+        # Under certain circumstances, we could end-up treating a model-class
+        # itself as a value. This check ensures that we drop the table alias
+        # into the query instead of trying to parameterize a model (for
+        # instance, when passing a model as a function argument).
+        if is_model(self.value):
+            with ctx.scope_column():
+                return ctx.sql(self.value)
+
         return ctx.value(self.value, self.converter)
 
 
@@ -6153,6 +6161,9 @@ def __len__(self):
     def __bool__(self): return True
     __nonzero__ = __bool__  # Python 2.
 
+    def __sql__(self, ctx):
+        return ctx.sql(self._meta.table)
+
 
 class _BoundModelsContext(_callable_context_manager):
     def __init__(self, models, database, bind_refs, bind_backrefs):

tests/model_sql.py

@@ -1076,3 +1076,21 @@ class Article(Model):
         self.assertSQL(aidx, (
             'CREATE UNIQUE INDEX IF NOT EXISTS "article_timestamp" '
             'ON "article" ("timestamp" DESC, ("flags" & ?))'), [4])
+
+
+class TestModelArgument(BaseTestCase):
+    database = SqliteDatabase(None)
+
+    def test_model_as_argument(self):
+        class Post(TestModel):
+            content = TextField()
+            timestamp = DateTimeField()
+            class Meta:
+                database = self.database
+
+        query = (Post
+                 .select(Post.id, fn.score(Post).alias('score'))
+                 .order_by(Post.timestamp))
+        self.assertSQL(query, (
+            'SELECT "t1"."id", score("t1") AS "score" '
+            'FROM "post" AS "t1" ORDER BY "t1"."timestamp"'), [])