fix(format): keep MODEL/AUDIT header dialect-agnostic (#5773)

format_model_expressions rendered every parsed expression with the target
dialect, including the SQLMesh-specific MODEL/AUDIT/METRIC header. That header
is SQLMesh DDL, not standard SQL, so transpiling it corrupts boolean properties:
under tsql (which has no TRUE keyword) a property like `allow_partials true`
was rewritten to `allow_partials (1 = 1)`.

Render meta expressions (is_meta_expression) with dialect=None so the header
stays as written, while the actual query/statement body still transpiles to the
target dialect.

Signed-off-by: Anas Khan <83116240+anxkhn@users.noreply.github.com>

Author: anxkhn

sqlmesh/core/dialect.py

@@ -783,7 +783,10 @@ def format_model_expressions(
         A string representing the formatted model.
     """
     if len(expressions) == 1 and is_meta_expression(expressions[0]):
-        return expressions[0].sql(pretty=True, dialect=dialect)
+        # Meta expressions (MODEL/AUDIT/METRIC) are SQLMesh DDL, not standard SQL,
+        # so they must never be transpiled to the target dialect (e.g. tsql would
+        # rewrite a boolean property like `allow_partials TRUE` to `(1 = 1)`).
+        return expressions[0].sql(pretty=True, dialect=None)
 
     if rewrite_casts:
 
@@ -815,7 +818,14 @@ def cast_to_colon(node: exp.Expr) -> exp.Expr:
         expressions = new_expressions
 
     return ";\n\n".join(
-        expression.sql(pretty=True, dialect=dialect, **kwargs) for expression in expressions
+        # Meta expressions (MODEL/AUDIT/METRIC) are SQLMesh DDL and must stay
+        # dialect-agnostic; only the actual query/statement expressions transpile.
+        expression.sql(
+            pretty=True,
+            dialect=None if is_meta_expression(expression) else dialect,
+            **kwargs,
+        )
+        for expression in expressions
     ).strip()
 
 

tests/core/test_dialect.py

@@ -230,6 +230,33 @@ def test_format_model_expressions():
   1::Int32 AS bar"""
     )
 
+    x = format_model_expressions(
+        parse(
+            """
+            MODEL(name a.b, kind FULL, dialect tsql, allow_partials true);
+            SELECT TRUE AS col, CAST(x AS INT) AS y FROM t
+            """
+        ),
+        dialect="tsql",
+    )
+    # The MODEL header is SQLMesh DDL and must not be transpiled: a boolean property
+    # such as `allow_partials true` must stay `TRUE`, not become tsql's `(1 = 1)`.
+    # The query body must still transpile to the target dialect.
+    assert (
+        x
+        == """MODEL (
+  name a.b,
+  kind FULL,
+  dialect tsql,
+  allow_partials TRUE
+);
+
+SELECT
+  1 AS col,
+  x::INTEGER AS y
+FROM t"""
+    )
+
     x = format_model_expressions(
         parse(
             """