Start adding operations that work and tests for them

django/db/migrations/operations/fields.py

@@ -0,0 +1,52 @@
+from .base import Operation
+
+
+class AddField(Operation):
+    """
+    Adds a field to a model.
+    """
+
+    def __init__(self, model_name, name, instance):
+        self.model_name = model_name
+        self.name = name
+        self.instance = instance
+
+    def state_forwards(self, app_label, state):
+        state.models[app_label, self.model_name.lower()].fields.append((self.name, self.instance))
+
+    def database_forwards(self, app_label, schema_editor, from_state, to_state):
+        app_cache = to_state.render()
+        model = app_cache.get_model(app_label, self.name)
+        schema_editor.add_field(model, model._meta.get_field_by_name(self.name))
+
+    def database_backwards(self, app_label, schema_editor, from_state, to_state):
+        app_cache = from_state.render()
+        model = app_cache.get_model(app_label, self.name)
+        schema_editor.remove_field(model, model._meta.get_field_by_name(self.name))
+
+
+class RemoveField(Operation):
+    """
+    Removes a field from a model.
+    """
+
+    def __init__(self, model_name, name):
+        self.model_name = model_name
+        self.name = name
+
+    def state_forwards(self, app_label, state):
+        new_fields = []
+        for name, instance in state.models[app_label, self.model_name.lower()].fields:
+            if name != self.name:
+                new_fields.append((name, instance))
+        state.models[app_label, self.model_name.lower()].fields = new_fields
+
+    def database_forwards(self, app_label, schema_editor, from_state, to_state):
+        app_cache = from_state.render()
+        model = app_cache.get_model(app_label, self.name)
+        schema_editor.remove_field(model, model._meta.get_field_by_name(self.name))
+
+    def database_backwards(self, app_label, schema_editor, from_state, to_state):
+        app_cache = to_state.render()
+        model = app_cache.get_model(app_label, self.name)
+        schema_editor.add_field(model, model._meta.get_field_by_name(self.name))