added unique columns constraints to Meta options

tests/test_unique_constraints.py

@@ -0,0 +1,53 @@
+import asyncio
+import sqlite3
+
+import databases
+import pytest
+import sqlalchemy
+from sqlalchemy.exc import IntegrityError
+
+import ormar
+from tests.settings import DATABASE_URL
+
+database = databases.Database(DATABASE_URL, force_rollback=True)
+metadata = sqlalchemy.MetaData()
+
+
+class Product(ormar.Model):
+    class Meta:
+        tablename = "products"
+        metadata = metadata
+        database = database
+        constraints = [ormar.UniqueColumns('name', 'company')]
+
+    id: ormar.Integer(primary_key=True)
+    name: ormar.String(max_length=100)
+    company: ormar.String(max_length=200)
+
+
+@pytest.fixture(scope="module")
+def event_loop():
+    loop = asyncio.get_event_loop()
+    yield loop
+    loop.close()
+
+
+@pytest.fixture(autouse=True, scope="module")
+async def create_test_database():
+    engine = sqlalchemy.create_engine(DATABASE_URL)
+    metadata.drop_all(engine)
+    metadata.create_all(engine)
+    yield
+    metadata.drop_all(engine)
+
+
+@pytest.mark.asyncio
+async def test_unique_columns():
+    async with database:
+        async with database.transaction(force_rollback=True):
+            await Product.objects.create(name='Cookies', company='Nestle')
+            await Product.objects.create(name='Mars', company='Mars')
+            await Product.objects.create(name='Mars', company='Nestle')
+
+            with pytest.raises((IntegrityError, sqlite3.IntegrityError)):
+                await Product.objects.create(name='Mars', company='Mars')