#283 index alphanumeric ordering for Tag's name (#286)

* Create AddIndexSQL class

* Rename expressions file

* Self-review fixes

catalog/models.py

@@ -13,7 +13,7 @@
 from django.utils.translation import ugettext_lazy as _
 from unidecode import unidecode
 
-from catalog.expressions import Substring
+from catalog.models_expressions import Substring
 
 SLUG_MAX_LENGTH = 50
 
@@ -229,7 +229,6 @@ def __str__(self):
 class TagQuerySet(models.QuerySet):
     # @todo #273:30m Apply new order_by_alphanumeric for SE/STB.
 
-    # @todo #273:60m Create an index for order_by_alphanumeric query.
     def order_by_alphanumeric(self):
         """Sort the Tag by name's alphabetic chars and then by numeric chars."""
         return self.annotate(

catalog/models_operations.py

@@ -0,0 +1,57 @@
+from django.db.migrations.operations.base import Operation
+
+# @todo #283:30m Group models.py, models_operations.py, models_expressions.py into the module.
+
+
+class AddIndexSQL(Operation):
+    """
+    Create a B-Tree index by given columns or expressions.
+
+    Docs: https://docs.djangoproject.com/en/1.11/ref/migration-operations/#writing-your-own
+    """
+
+    reduces_to_sql = True
+    reversible = True
+
+    def __init__(self, name, columns, model_name):
+        self.name = name
+        self.columns = columns
+        self.model_name = model_name
+
+    def state_forwards(self, app_label, state):
+        """We have to implement this method for Operation interface."""
+
+    def _index_name(self, table_name):
+        return f'{table_name}_{self.name}_idx'
+
+    def database_forwards(self, app_label, schema_editor, _, to_state):
+        to_model = to_state.apps.get_model(app_label, self.model_name)
+        if self.allow_migrate_model(schema_editor.connection.alias, to_model):
+            table_name = to_model._meta.db_table
+            schema_editor.execute(
+                f'CREATE INDEX {self._index_name(table_name)} ON {table_name}'
+                f'({", ".join(self.columns)});'
+            )
+
+    def database_backwards(self, app_label, schema_editor, from_state, _):
+        from_model = from_state.apps.get_model(app_label, self.model_name)
+        if self.allow_migrate_model(schema_editor.connection.alias, from_model):
+            table_name = from_model._meta.db_table
+            schema_editor.execute(
+                f'DROP INDEX {self._index_name(table_name)};'
+            )
+
+    def describe(self):
+        return f'Create index {self.name} for {self.model_name}'
+
+# Django doesn't provide ability to add hooks to makemigrations.
+# So we have to create migration files and add operations for
+# abstract classes (like Tag) manually.
+index_alphanumeric_tag_name = AddIndexSQL(
+    name='alphanumeric_name',
+    columns=[
+        "substring(name, '[a-zA-Zа-яА-Я]+')",
+        "(substring(name, '[0-9]+\.?[0-9]*')::float)"
+    ],
+    model_name='tag',
+)