fixes prisma/prisma#2387

libs/datamodel/core/src/validator/validate.rs

@@ -506,6 +506,14 @@ impl<'a> Validator<'a> {
                     }
                 }
 
+                if rel_info.fields.len() != rel_info.to_fields.len() {
+                    errors.push(DatamodelError::new_directive_validation_error(
+                        &format!("You must specify the same number of fields in `fields` and `references`.",),
+                        RELATION_DIRECTIVE_NAME,
+                        ast_field.span.clone(),
+                    ));
+                }
+
                 if !fields_with_wrong_type.is_empty() && !errors.has_errors() {
                     // don't output too much errors
                     errors.append_vec(fields_with_wrong_type);

libs/datamodel/core/tests/directives/relations_new.rs

@@ -325,6 +325,31 @@ fn relation_must_error_when_types_of_base_field_and_referenced_field_do_not_matc
     errors.assert_is(DatamodelError::new_directive_validation_error("The type of the field `userId` in the model `Post` is not matching the type of the referenced field `id` in model `User`.","@relation", Span::new(204, 265)));
 }
 
+#[test]
+fn relation_must_error_when_number_of_fields_and_references_is_not_equal() {
+    let dml = r#"
+    model User {
+        id        Int @id
+        firstName String
+        posts     Post[]
+    }
+
+    model Post {
+        id       Int     @id
+        userId   Int
+        userName String
+        user     User    @relation(fields: [userId, userName], references: [id])
+    }
+    "#;
+
+    let errors = parse_error(dml);
+    errors.assert_is(DatamodelError::new_directive_validation_error(
+        "You must specify the same number of fields in `fields` and `references`.",
+        "@relation",
+        Span::new(200, 273),
+    ));
+}
+
 #[test]
 fn relation_must_succeed_when_type_alias_is_used_for_referenced_field() {
     let dml = r#"