use native decimal (numeric) type

* improves precision for decimals
* not supported by aiosqlite

Author: Jakub Urban

databases/backends/mysql.py

@@ -23,6 +23,7 @@ def __init__(
         self._database_url = DatabaseURL(database_url)
         self._options = options
         self._dialect = pymysql.dialect(paramstyle="pyformat")
+        self._dialect.supports_native_decimal = True
         self._pool = None
 
     def _get_connection_kwargs(self) -> dict:

databases/backends/postgres.py

@@ -36,6 +36,7 @@ def _get_dialect(self) -> Dialect:
         dialect._backslash_escapes = False
         dialect.supports_sane_multi_rowcount = True  # psycopg 2.0.9+
         dialect._has_native_hstore = True
+        dialect.supports_native_decimal = True
 
         return dialect
 

databases/backends/sqlite.py

@@ -22,6 +22,8 @@ def __init__(
         self._database_url = DatabaseURL(database_url)
         self._options = options
         self._dialect = pysqlite.dialect(paramstyle="qmark")
+        # aiosqlite does not support decimals
+        self._dialect.supports_native_decimal = False
         self._pool = SQLitePool(self._database_url, **self._options)
 
     async def connect(self) -> None:

tests/test_databases.py

@@ -1,5 +1,6 @@
 import asyncio
 import datetime
+import decimal
 import functools
 import os
 
@@ -61,6 +62,14 @@ def process_result_value(self, value, dialect):
     sqlalchemy.Column("published", MyEpochType),
 )
 
+# Used to test Numeric
+prices = sqlalchemy.Table(
+    "prices",
+    metadata,
+    sqlalchemy.Column("id", sqlalchemy.Integer, primary_key=True),
+    sqlalchemy.Column("price", sqlalchemy.Numeric(precision=30, scale=20)),
+)
+
 
 @pytest.fixture(autouse=True, scope="module")
 def create_test_database():
@@ -456,6 +465,33 @@ async def test_datetime_field(database_url):
             assert results[0]["published"] == now
 
 
+@pytest.mark.parametrize("database_url", DATABASE_URLS)
+@async_adapter
+async def test_decimal_field(database_url):
+    """
+    Test Decimal (NUMERIC) columns, to ensure records are coerced to/from proper Python types.
+    """
+
+    async with Database(database_url) as database:
+        async with database.transaction(force_rollback=True):
+            price = decimal.Decimal("0.700000000000001")
+
+            # execute()
+            query = prices.insert()
+            values = {"price": price}
+            await database.execute(query, values)
+
+            # fetch_all()
+            query = prices.select()
+            results = await database.fetch_all(query=query)
+            assert len(results) == 1
+            if database_url.startswith("sqlite"):
+                # aiosqlite does not support native decimals --> a roud-off error is expected
+                assert results[0]["price"] == pytest.approx(price)
+            else:
+                assert results[0]["price"] == price
+
+
 @pytest.mark.parametrize("database_url", DATABASE_URLS)
 @async_adapter
 async def test_json_field(database_url):