Types: Compile Float without a precision to DOUBLE

CHANGES.md

@@ -1,6 +1,11 @@
 # Changelog
 
 ## Unreleased
+- BREAKING: Types: The generic `Float` without a precision now compiles to
+  `DOUBLE` instead of `FLOAT`, CrateDB's 4-byte `REAL`, which rounded Python
+  floats to 24 bits. This matches SQLAlchemy's documented behaviour on
+  backends that distinguish the two. `Float(precision<=24)`, `REAL` and the
+  uppercase `FLOAT` type still render 4-byte columns
 - Types: Fixed `CLOB`, `NCHAR`, `NVARCHAR`, `DATETIME`, and `DATE` compiling to
   type names CrateDB cannot parse. They now map to `STRING`, `CHAR`, `VARCHAR`,
   and `TIMESTAMP` respectively, matching their generic lower-case counterparts

src/sqlalchemy_cratedb/compiler.py

@@ -328,6 +328,20 @@ def visit_FLOAT(self, type_, **kw):
             return "DOUBLE"
         return "FLOAT"
 
+    def visit_float(self, type_, **kw):
+        """
+        The generic `Float` without a precision is an 8-byte `DOUBLE`.
+
+        CrateDB's `FLOAT` is the 4-byte `REAL`, which rounds Python floats to
+        24 bits of precision. SQLAlchemy documents a precision-less `Float` as
+        8-byte on backends that distinguish the two, such as PostgreSQL and
+        MSSQL. A 4-byte column is still available through `Float(precision=24)`
+        or lower, `REAL`, or the uppercase `FLOAT` type.
+        """
+        if type_.precision is None:
+            return "DOUBLE"
+        return self.visit_FLOAT(type_, **kw)
+
     def visit_JSON(self, type_, **kw):
         return "OBJECT"
 

tests/compiler_test.py

@@ -23,12 +23,14 @@
 from unittest import TestCase, mock, skipIf
 from unittest.mock import MagicMock, patch
 
+import pytest
 import sqlalchemy as sa
 from crate.client.cursor import Cursor
 from sqlalchemy.dialects.postgresql import insert as pg_insert
 from sqlalchemy.sql import Update, text
 
 from sqlalchemy_cratedb.compiler import crate_before_execute
+from sqlalchemy_cratedb.dialect import CrateDialect
 from tests.settings import crate_host
 from tests.util import ExtraAssertions
 
@@ -605,3 +607,26 @@ def test_ddl_with_jsonb_columns(self):
 
         """),
         )
+
+
+FLOAT_DDL_CASES = [
+    (sa.Float(), "DOUBLE"),
+    (sa.Float(asdecimal=True), "DOUBLE"),
+    (sa.Float(precision=53), "DOUBLE"),
+    (sa.Float(precision=25), "DOUBLE"),
+    (sa.Float(precision=24), "FLOAT"),
+    (sa.Float(precision=10), "FLOAT"),
+    (sa.FLOAT(), "FLOAT"),
+    (sa.REAL(), "REAL"),
+]
+# `sa.Double` is only available on SQLAlchemy>=2.0.
+if hasattr(sa, "Double"):
+    FLOAT_DDL_CASES.append((sa.Double(), "DOUBLE"))
+
+
+@pytest.mark.parametrize("type_,expected", FLOAT_DDL_CASES)
+def test_float_ddl(type_, expected):
+    """
+    A precision-less generic `Float` is 8-byte; 4-byte types stay available.
+    """
+    assert type_.compile(dialect=CrateDialect()) == expected

tests/create_table_test.py

@@ -73,7 +73,7 @@ class User(self.Base):
                 "\n\tshort_col SHORT, "
                 "\n\tdatetime_col TIMESTAMP WITHOUT TIME ZONE, "
                 "\n\tdate_col TIMESTAMP, "
-                "\n\tfloat_col FLOAT, "
+                "\n\tfloat_col DOUBLE, "
                 "\n\tdecimal_col NUMERIC(10, 2), "
                 "\n\tPRIMARY KEY (string_col)\n)\n\n"
             ),