fix(mysql): improve diffing of defaults for JSON columns

Closes #4926

packages/knex/src/schema/SchemaComparator.ts

@@ -562,8 +562,8 @@ export class SchemaComparator {
     }
 
     if (to.mappedType instanceof JsonType) {
-      const defaultValueFrom = parseJsonSafe(from.default.replace(/^'(.*)'$/, '$1'));
-      const defaultValueTo = parseJsonSafe(to.default?.replace(/^'(.*)'$/, '$1'));
+      const defaultValueFrom = parseJsonSafe(from.default.replace(/^(_\w+\\)?'(.*?)\\?'$/, '$2'));
+      const defaultValueTo = parseJsonSafe(to.default?.replace(/^\(?'(.*?)'\)?$/, '$1'));
 
       return Utils.equals(defaultValueFrom, defaultValueTo);
     }

tests/features/schema-generator/json-diffing.mysql.test.ts

@@ -0,0 +1,34 @@
+import { Entity, PrimaryKey, Property } from '@mikro-orm/core';
+import { MikroORM } from '@mikro-orm/mysql';
+
+@Entity()
+class User {
+
+  @PrimaryKey()
+  id!: number;
+
+  @Property()
+  name!: string;
+
+  @Property({ type: 'json', defaultRaw: `('{"a": 1}')` })
+  data!: any;
+
+}
+
+let orm: MikroORM;
+
+beforeAll(async () => {
+  orm = await MikroORM.init({
+    entities: [User],
+    dbName: `4926`,
+    port: 3308,
+  });
+  await orm.schema.refreshDatabase();
+});
+
+afterAll(() => orm.close(true));
+
+test('default values on json columns', async () => {
+  const diff1 = await orm.schema.getUpdateSchemaSQL({ wrap: false });
+  expect(diff1).toBe('');
+});