fix(core): infer property type from columnType for non-inferrable t…

…ypes (e.g. unions with `null`)

Closes #4833

packages/core/src/metadata/MetadataDiscovery.ts

@@ -1079,6 +1079,11 @@ export class MetadataDiscovery {
       prop.scale ??= pk.scale;
     });
 
+    if (prop.reference === ReferenceType.SCALAR && prop.type == null && prop.columnTypes) {
+      const mappedType = this.getMappedType(prop);
+      prop.type = mappedType.compareAsType();
+    }
+
     if (prop.columnTypes || !this.schemaHelper) {
       return;
     }

packages/core/src/metadata/ReflectMetadataProvider.ts

@@ -19,7 +19,9 @@ export class ReflectMetadataProvider extends MetadataProvider {
     // Instead of requiring the type everywhere, we default to string, which maintains the behaviour,
     // as we were mapping it to UnknownType which is a string. This is to prevent defaulting to JSON
     // column type, which can be often hard to revert and cause hard to understand issues with PKs.
-    if (prop.reference === ReferenceType.SCALAR && type === Object) {
+    // If there are explicitly provided `columnTypes`, we use those instead for the inference, this way
+    // we can have things like `columnType: 'timestamp'` be respected as `type: 'Date'`.
+    if (prop.reference === ReferenceType.SCALAR && type === Object && !prop.columnTypes) {
       type = String;
     }
 

tests/issues/GH4833.test.ts

@@ -0,0 +1,35 @@
+import { Entity, PrimaryKey, Property } from '@mikro-orm/core';
+import { MikroORM } from '@mikro-orm/sqlite';
+
+@Entity()
+class Foo {
+
+  @PrimaryKey()
+  id!: number;
+
+  @Property()
+  bar!: string;
+
+  @Property({ columnType: 'timestamp(3)', nullable: true })
+  createdAt: Date | null = null;
+
+}
+
+let orm: MikroORM;
+
+beforeAll(async () => {
+  orm = await MikroORM.init({
+    dbName: ':memory:',
+    entities: [Foo],
+  });
+  await orm.schema.createSchema();
+});
+
+afterAll(async () => {
+  await orm.close(true);
+});
+
+test(`GH issue 1352`, async () => {
+  const foo = orm.em.create(Foo, { bar: 'baz' });
+  orm.em.assign(foo, { createdAt: new Date() });
+});