Better mixins support (#80)

This addresses #75 by looking at all columns available in mro, not just topmost model.

It doesn't address the rest of the "mixins" story such as `declared_attr`.

Author: rafales

sqlmypy.py

@@ -209,12 +209,13 @@ def model_hook(ctx: FunctionContext) -> Type:
     # Collect column names and types defined in the model
     # TODO: cache this?
     expected_types = {}  # type: Dict[str, Type]
-    for name, sym in model.names.items():
-        if isinstance(sym.node, Var) and isinstance(sym.node.type, Instance):
-            tp = sym.node.type
-            if tp.type.fullname() in (COLUMN_NAME, RELATIONSHIP_NAME):
-                assert len(tp.args) == 1
-                expected_types[name] = tp.args[0]
+    for cls in model.mro[::-1]:
+        for name, sym in cls.names.items():
+            if isinstance(sym.node, Var) and isinstance(sym.node.type, Instance):
+                tp = sym.node.type
+                if tp.type.fullname() in (COLUMN_NAME, RELATIONSHIP_NAME):
+                    assert len(tp.args) == 1
+                    expected_types[name] = tp.args[0]
 
     assert len(ctx.arg_names) == 1  # only **kwargs in generated __init__
     assert len(ctx.arg_types) == 1

test/test-data/sqlalchemy-plugin-features.test

@@ -50,6 +50,47 @@ class Base:
     ...
 [out]
 
+[case testModelInitMixin]
+
+from sqlalchemy import Column, Integer, String, DateTime
+from sqlalchemy.ext.declarative import declarative_base
+
+Base = declarative_base()
+
+class HasId:
+    id = Column(Integer, primary_key=True)
+
+class User(Base, HasId):
+    __tablename__ = 'users'
+
+    name = Column(String, nullable=False)
+
+user = User(id=123, name="John Doe")
+reveal_type(user.id)  # N: Revealed type is 'builtins.int*'
+[out]
+
+[case testModelInitProperMro]
+
+from sqlalchemy import Column, Integer, String, DateTime
+from sqlalchemy.ext.declarative import declarative_base
+
+Base = declarative_base()
+
+class Defaults:
+    id = Column(Integer, primary_key=True)
+
+class User(Base, Defaults):
+    __tablename__ = 'users'
+
+    # By default mypy will complain about Column[str] not being compatible with Column[int].
+    # Adding "ignore" should allow us to override column type.
+    id = Column(String, primary_key=True)  # type: ignore
+    name = Column(String, nullable=False)
+
+User(id='stringish-id')
+User(id=123)  # E: Incompatible type for "id" of "User" (got "int", expected "str")
+[out]
+
 [case testModelInitRelationship]
 from typing import TYPE_CHECKING, List