test: add tests for self relationships (#10071)

Initially I thought that we will have to add special checks to allow relationships referencing itself. However, it turned out that not to be the case. Instead, it had more to do with how self relationships are defined in Mikro ORM.

In case of `belongsTo` relationship we have to define the other side as well either as a `hasMany` or `hasOne`. For example:

**❌ The following code will fail, because no children are defined for the parent**
```ts
const user = model.define("user", {
  id: model.number(),
  username: model.text(),
  parent: model.belongsTo(() => user)
})
```

**✅ Addition of children relationship will make things work**
```ts
const user = model.define("user", {
  id: model.number(),
  username: model.text(),
  parent: model.belongsTo(() => user, { mappedBy: "children" }),
  children: model.hasMany(() => user, { mappedBy: "parent" }),
})
```

We can see the similar setup here with our `ProductCategory` MikroORM entity. https://github.com/medusajs/medusa/blob/develop/packages/modules/product/src/models/product-category.ts#L87-L94

@adrien2p Correct me if I am wrong. But I have added the tests for now so that we know the behavior of self relationships

packages/core/utils/src/dml/__tests__/entity-builder.spec.ts

@@ -4702,6 +4702,236 @@ describe("Entity builder", () => {
         },
       })
     })
+
+    test("define entity with relationship to itself via hasMany", () => {
+      const user = model.define("user", {
+        id: model.number(),
+        username: model.text(),
+        parent: model.belongsTo(() => user, { mappedBy: "children" }),
+        children: model.hasMany(() => user, { mappedBy: "parent" }),
+      })
+
+      const [User] = toMikroOrmEntities([user])
+
+      expectTypeOf(new User()).toMatchTypeOf<{
+        id: number
+        username: string
+        deleted_at: Date | null
+        parent: {
+          id: number
+          username: string
+          deleted_at: Date | null
+        }
+        children: {
+          id: number
+          username: string
+          deleted_at: Date | null
+        }[]
+      }>()
+
+      const metaData = MetadataStorage.getMetadataFromDecorator(User)
+      expect(metaData.className).toEqual("User")
+      expect(metaData.path).toEqual("User")
+      expect(metaData.properties).toEqual({
+        id: {
+          reference: "scalar",
+          type: "number",
+          columnType: "integer",
+          name: "id",
+          fieldName: "id",
+          nullable: false,
+          getter: false,
+          setter: false,
+        },
+        username: {
+          reference: "scalar",
+          type: "string",
+          columnType: "text",
+          name: "username",
+          fieldName: "username",
+          nullable: false,
+          getter: false,
+          setter: false,
+        },
+        parent: {
+          name: "parent",
+          reference: "m:1",
+          entity: "User",
+          persist: false,
+          nullable: false,
+        },
+        parent_id: {
+          name: "parent_id",
+          reference: "m:1",
+          entity: "User",
+          columnType: "text",
+          fieldName: "parent_id",
+          mapToPk: true,
+          nullable: false,
+          onDelete: undefined,
+          isForeignKey: true,
+        },
+        children: {
+          cascade: undefined,
+          entity: "User",
+          mappedBy: "parent",
+          name: "children",
+          orphanRemoval: true,
+          reference: "1:m",
+        },
+        created_at: {
+          reference: "scalar",
+          type: "date",
+          columnType: "timestamptz",
+          name: "created_at",
+          fieldName: "created_at",
+          defaultRaw: "now()",
+          onCreate: expect.any(Function),
+          nullable: false,
+          getter: false,
+          setter: false,
+        },
+        updated_at: {
+          reference: "scalar",
+          type: "date",
+          columnType: "timestamptz",
+          name: "updated_at",
+          fieldName: "updated_at",
+          defaultRaw: "now()",
+          onCreate: expect.any(Function),
+          onUpdate: expect.any(Function),
+          nullable: false,
+          getter: false,
+          setter: false,
+        },
+        deleted_at: {
+          reference: "scalar",
+          type: "date",
+          columnType: "timestamptz",
+          name: "deleted_at",
+          fieldName: "deleted_at",
+          nullable: true,
+          getter: false,
+          setter: false,
+        },
+      })
+    })
+
+    test("define relationship with self via hasOne", () => {
+      const user = model.define("user", {
+        id: model.number(),
+        username: model.text(),
+        parent: model.belongsTo(() => user, { mappedBy: "child" }),
+        child: model.hasOne(() => user, { mappedBy: "parent" }),
+      })
+
+      const [User] = toMikroOrmEntities([user])
+
+      expectTypeOf(new User()).toMatchTypeOf<{
+        id: number
+        username: string
+        deleted_at: Date | null
+        parent: {
+          id: number
+          username: string
+          deleted_at: Date | null
+        }
+        child: {
+          id: number
+          username: string
+          deleted_at: Date | null
+        }
+      }>()
+
+      const metaData = MetadataStorage.getMetadataFromDecorator(User)
+      expect(metaData.className).toEqual("User")
+      expect(metaData.path).toEqual("User")
+      expect(metaData.properties).toEqual({
+        id: {
+          reference: "scalar",
+          type: "number",
+          columnType: "integer",
+          name: "id",
+          fieldName: "id",
+          nullable: false,
+          getter: false,
+          setter: false,
+        },
+        username: {
+          reference: "scalar",
+          type: "string",
+          columnType: "text",
+          name: "username",
+          fieldName: "username",
+          nullable: false,
+          getter: false,
+          setter: false,
+        },
+        parent: {
+          name: "parent",
+          mappedBy: "child",
+          reference: "1:1",
+          entity: "User",
+          nullable: false,
+          onDelete: undefined,
+          owner: true,
+        },
+        parent_id: {
+          name: "parent_id",
+          type: "string",
+          columnType: "text",
+          isForeignKey: true,
+          persist: false,
+          reference: "scalar",
+          getter: false,
+          setter: false,
+          nullable: false,
+        },
+        child: {
+          cascade: undefined,
+          entity: "User",
+          mappedBy: "parent",
+          name: "child",
+          nullable: false,
+          reference: "1:1",
+        },
+        created_at: {
+          reference: "scalar",
+          type: "date",
+          columnType: "timestamptz",
+          name: "created_at",
+          fieldName: "created_at",
+          defaultRaw: "now()",
+          onCreate: expect.any(Function),
+          nullable: false,
+          getter: false,
+          setter: false,
+        },
+        updated_at: {
+          reference: "scalar",
+          type: "date",
+          columnType: "timestamptz",
+          name: "updated_at",
+          fieldName: "updated_at",
+          defaultRaw: "now()",
+          onCreate: expect.any(Function),
+          onUpdate: expect.any(Function),
+          nullable: false,
+          getter: false,
+          setter: false,
+        },
+        deleted_at: {
+          reference: "scalar",
+          type: "date",
+          columnType: "timestamptz",
+          name: "deleted_at",
+          fieldName: "deleted_at",
+          nullable: true,
+          getter: false,
+          setter: false,
+        },
+      })
+    })
   })
 
   describe("Entity builder | manyToMany", () => {