Allow setting min_size and max_size in postgres DSN

Overtakes encode#129
Closes encode#78

databases/backends/postgres.py

@@ -62,7 +62,14 @@ def _get_connection_kwargs(self) -> dict:
     async def connect(self) -> None:
         assert self._pool is None, "DatabaseBackend is already running"
         kwargs = self._get_connection_kwargs()
-        self._pool = await asyncpg.create_pool(str(self._database_url), **kwargs)
+        self._pool = await asyncpg.create_pool(
+            host=self._database_url.hostname,
+            port=self._database_url.port,
+            user=self._database_url.username,
+            password=self._database_url.password,
+            database=self._database_url.database,
+            **kwargs,
+        )
 
     async def disconnect(self) -> None:
         assert self._pool is not None, "DatabaseBackend is not running"

tests/test_connection_options.py

@@ -13,6 +13,12 @@ def test_postgres_pool_size():
     assert kwargs == {"min_size": 1, "max_size": 20}
 
 
+def test_postgres_pool_size_connect():
+    backend = PostgresBackend("postgres://localhost/database?min_size=1&max_size=20")
+    await backend.connect()
+    await backend.disconnect()
+
+
 def test_postgres_explicit_pool_size():
     backend = PostgresBackend("postgres://localhost/database", min_size=1, max_size=20)
     kwargs = backend._get_connection_kwargs()