test: check STI type setting discriminator manually

Related to: typeorm#9033

test/github-issues/9033/entity/Animal.ts

@@ -0,0 +1,22 @@
+import {
+    Column,
+    Entity,
+    PrimaryGeneratedColumn,
+    TableInheritance,
+} from "../../../../src"
+
+@Entity("animal")
+@TableInheritance({ column: { type: "varchar", name: "type" } })
+export class AnimalEntity {
+    @PrimaryGeneratedColumn()
+    id: number
+
+    @Column({ type: "varchar" })
+    name: string
+
+    @Column({
+        name: "type",
+        type: "varchar",
+    })
+    type: string
+}

test/github-issues/9033/entity/Cat.ts

@@ -0,0 +1,10 @@
+import { ChildEntity, Column } from "../../../../src"
+
+import { AnimalEntity } from "./Animal"
+
+@ChildEntity("cat")
+export class CatEntity extends AnimalEntity {
+    // Cat stuff
+    @Column()
+    livesLeft: number
+}

test/github-issues/9033/issue-9033.ts

@@ -0,0 +1,48 @@
+import "reflect-metadata"
+import {
+    createTestingConnections,
+    closeTestingConnections,
+    reloadTestingDatabases,
+} from "../../utils/test-utils"
+import { DataSource } from "../../../src/data-source/index"
+import { expect } from "chai"
+
+import { CatEntity } from "./entity/Cat"
+import { AnimalEntity } from "./entity/Animal"
+
+describe("github issues > #9033 Cannot manually insert type in discriminator column of parent entity class when\
+using single table inheritance when creating instance of parent entity", () => {
+    let dataSources: DataSource[]
+    before(
+        async () =>
+            (dataSources = await createTestingConnections({
+                entities: [__dirname + "/entity/*{.js,.ts}"],
+                schemaCreate: true,
+                dropSchema: true,
+            })),
+    )
+    beforeEach(() => reloadTestingDatabases(dataSources))
+    after(() => closeTestingConnections(dataSources))
+
+    it("is possible to set the discriminator column manually on the base entity", () =>
+        Promise.all(
+            dataSources.map(async (dataSource) => {
+                const entityManager = dataSource.createEntityManager()
+                const animalRepo = entityManager.getRepository(AnimalEntity)
+
+                // Create a base class entity while manually setting discriminator.
+                const maybeACat = animalRepo.create({
+                    name: "i-am-maybe-a-cat",
+                    type: "cat", // This is the discriminator for `CatEntity`.
+                })
+                await entityManager.save(maybeACat)
+
+                // Load the animal / cat from the database.
+                const animals = await animalRepo.find()
+                expect(animals.length).to.equal(1)
+
+                expect(animals[0]).to.be.instanceOf(CatEntity)
+                expect(animals[0].type).to.equal("cat")
+            }),
+        ))
+})