use djangos method of wiping sqlite3 tables

Django uses a reference graph to determine the order
in table deletions that occur. Do the same here.

This commit also adds in the `REGEXP` sqlite function,
exactly how Django uses it in its reference graphing.

Signed-off-by: Kevin Morris <kevr@0cost.org>

aurweb/testing/__init__.py

@@ -1,6 +1,28 @@
+from itertools import chain
+
 import aurweb.db
 
 
+def references_graph(table):
+    """ Taken from Django's sqlite3/operations.py. """
+    query = """
+    WITH tables AS (
+    SELECT :table name
+    UNION
+    SELECT sqlite_master.name
+    FROM sqlite_master
+    JOIN tables ON (sql REGEXP :regexp_1 || tables.name || :regexp_2)
+    ) SELECT name FROM tables;
+    """
+    params = {
+        "table": table,
+        "regexp_1": r'(?i)\s+references\s+("|\')?',
+        "regexp_2": r'("|\')?\s*\(',
+    }
+    cursor = aurweb.db.session.execute(query, params=params)
+    return [row[0] for row in cursor.fetchall()]
+
+
 def setup_test_db(*args):
     """ This function is to be used to setup a test database before
     using it. It takes a variable number of table strings, and for
@@ -25,8 +47,22 @@ def setup_test_db(*args):
     aurweb.db.get_engine()
 
     tables = list(args)
+
+    db_backend = aurweb.config.get("database", "backend")
+
+    if db_backend != "sqlite":
+        aurweb.db.session.execute("SET FOREIGN_KEY_CHECKS = 0")
+    else:
+        # We're using sqlite, setup tables to be deleted without violating
+        # foreign key constraints by graphing references.
+        tables = set(chain.from_iterable(
+            references_graph(table) for table in tables))
+
     for table in tables:
         aurweb.db.session.execute(f"DELETE FROM {table}")
 
+    if db_backend != "sqlite":
+        aurweb.db.session.execute("SET FOREIGN_KEY_CHECKS = 1")
+
     # Expunge all objects from SQLAlchemy's IdentityMap.
     aurweb.db.session.expunge_all()