Implement column defaults for INSERT/UPDATE

databases/core.py

@@ -9,6 +9,8 @@
 
 from sqlalchemy import text
 from sqlalchemy.sql import ClauseElement
+from sqlalchemy.sql.dml import ValuesBase
+from sqlalchemy.sql.expression import type_coerce
 
 from databases.importer import import_from_string
 from databases.interfaces import ConnectionBackend, DatabaseBackend, TransactionBackend
@@ -294,11 +296,51 @@ def _build_query(
             query = text(query)
 
             return query.bindparams(**values) if values is not None else query
-        elif values:
+
+        # 2 paths where we apply column defaults:
+        # - values are supplied (the object must be a ValuesBase)
+        # - values is None but the object is a ValuesBase
+        if values is not None and not isinstance(query, ValuesBase):
+            raise TypeError("values supplied but query doesn't support .values()")
+
+        if values is not None or isinstance(query, ValuesBase):
+            values = Connection._apply_column_defaults(query, values)
             return query.values(**values)
 
         return query
 
+    @staticmethod
+    def _apply_column_defaults(query: ValuesBase, values: dict = None) -> dict:
+        """Add default values from the table of a query."""
+        new_values = {}
+        values = values or {}
+
+        for column in query.table.c:
+            if column.name in values:
+                continue
+
+            if column.default:
+                default = column.default
+
+                if default.is_sequence:  # pragma: no cover
+                    # TODO: support sequences
+                    continue
+                elif default.is_callable:
+                    value = default.arg(FakeExecutionContext())
+                elif default.is_clause_element:  # pragma: no cover
+                    # TODO: implement clause element
+                    # For this, the _build_query method needs to
+                    # become an instance method so that it can access
+                    # self._connection.
+                    continue
+                else:
+                    value = default.arg
+
+                new_values[column.name] = value
+
+        new_values.update(values)
+        return new_values
+
 
 class Transaction:
     def __init__(
@@ -489,3 +531,20 @@ def __repr__(self) -> str:
 
     def __eq__(self, other: typing.Any) -> bool:
         return str(self) == str(other)
+
+
+class FakeExecutionContext:
+    """
+    This is an object that raises an error when one of its properties are
+    attempted to be accessed. Because we're not _really_ using SQLAlchemy
+    (besides using its query builder), we can't pass a real ExecutionContext
+    to ColumnDefault objects. This class makes it so that any attempts to
+    access the execution context argument by a column default callable
+    blows up loudly and clearly.
+    """
+
+    def __getattr__(self, _: str) -> typing.NoReturn:  # pragma: no cover
+        raise NotImplementedError(
+            "Databases does not have a real SQLAlchemy ExecutionContext "
+            "implementation."
+        )

tests/test_databases.py

@@ -70,6 +70,17 @@ def process_result_value(self, value, dialect):
     sqlalchemy.Column("price", sqlalchemy.Numeric(precision=30, scale=20)),
 )
 
+# Used to test column default values
+timestamps = sqlalchemy.Table(
+    "timestamps",
+    metadata,
+    sqlalchemy.Column("id", sqlalchemy.Integer, primary_key=True),
+    sqlalchemy.Column(
+        "timestamp", sqlalchemy.DateTime, default=datetime.datetime.now, nullable=False
+    ),
+    sqlalchemy.Column("priority", sqlalchemy.Integer, default=0, nullable=False),
+)
+
 
 @pytest.fixture(autouse=True, scope="module")
 def create_test_database():
@@ -925,3 +936,33 @@ async def test_column_names(database_url, select_query):
             assert sorted(results[0].keys()) == ["completed", "id", "text"]
             assert results[0]["text"] == "example1"
             assert results[0]["completed"] == True
+
+
+@pytest.mark.parametrize("database_url", DATABASE_URLS)
+@async_adapter
+async def test_column_defaults(database_url):
+    """
+    Test correct usage of column defaults.
+    """
+    async with Database(database_url) as database:
+        async with database.transaction(force_rollback=True):
+            # with just defaults
+            query = timestamps.insert()
+            await database.execute(query)
+            results = await database.fetch_all(query=timestamps.select())
+            assert len(results) == 1
+            await database.execute(timestamps.delete())
+
+            # with default value overridden
+            dt = datetime.datetime.now()
+            dt -= datetime.timedelta(seconds=10, microseconds=dt.microsecond)
+            values = {"timestamp": dt}
+            await database.execute(query, values)
+            results = await database.fetch_all(timestamps.select())
+            assert len(results) == 1
+            assert results[0]["timestamp"] == dt
+
+            # testing invalid passing of values with non-ValuesBase
+            # argument
+            with pytest.raises(TypeError, match=r".*support \.values\(\).*"):
+                await database.execute(timestamps.select(), {})