- A warning is emitted when :func:`.cast` is used with the MySQL

dialect on a type where MySQL does not support CAST; MySQL only
supports CAST on a subset of datatypes.   SQLAlchemy has for a long
time just omitted the CAST for unsupported types in the case of
MySQL.  While we don't want to change this now, we emit a warning
to show that it's taken place.   A warning is also emitted when
a CAST is used with an older MySQL version (< 4) that doesn't support
CAST at all, it's skipped in this case as well.
fixes #3237

1 files changed, 7 insertions, 0 deletions

diff --git a/lib/sqlalchemy/dialects/mysql/base.py b/lib/sqlalchemy/dialects/mysql/base.py
index cbb108f5e..8d62bae02 100644
--- a/lib/sqlalchemy/dialects/mysql/base.py
+++ b/lib/sqlalchemy/dialects/mysql/base.py
@@ -1710,10 +1710,17 @@ class MySQLCompiler(compiler.SQLCompiler):
     def visit_cast(self, cast, **kwargs):
         # No cast until 4, no decimals until 5.
         if not self.dialect._supports_cast:
+            util.warn(
+                "Current MySQL version does not support "
+                "CAST; the CAST will be skipped.")
             return self.process(cast.clause.self_group())
 
         type_ = self.process(cast.typeclause)
         if type_ is None:
+            util.warn(
+                "Datatype %s does not support CAST on MySQL; "
+                "the CAST will be skipped." %
+                self.dialect.type_compiler.process(cast.typeclause.type))
             return self.process(cast.clause.self_group())
 
         return 'CAST(%s AS %s)' % (self.process(cast.clause), type_)