sql/fake.py: Some fake objects/methods to calm the code

sql/fake.py contains some fake classes and functions to
calm the rest of the code base. Or provide super minimal
implementations.

Currently:
- ObjectId "class": It's a function mostly doing
  int(first_arg) to convert string primary keys into
  integer primary keys.
- InvalidId exception
- DESCENING "constant"

mediagoblin/db/sql/fake.py

@@ -0,0 +1,28 @@
+"""
+This module contains some fake classes and functions to
+calm the rest of the code base. Or provide super minimal
+implementations.
+
+Currently:
+- ObjectId "class": It's a function mostly doing
+  int(init_arg) to convert string primary keys into
+  integer primary keys.
+- InvalidId exception
+- DESCENDING "constant"
+"""
+    
+
+DESCENDING = object()  # a unique object for this "constant"
+
+
+class InvalidId(Exception):
+    pass
+
+
+def ObjectId(value=None):
+    if value is None:
+        return None
+    try:
+        return int(value)
+    except ValueError:
+        raise InvalidId("%r is an invalid id" % value)