test: Add test to prove that issue #2331 is resolved (#9688)

test/github-issues/2331/entity/Post.ts

@@ -0,0 +1,13 @@
+import { BaseEntity, Entity, Column, PrimaryColumn } from "../../../../src"
+
+@Entity()
+export class Post extends BaseEntity {
+    @PrimaryColumn()
+    id: number
+
+    @Column({ type: "varchar" })
+    title: string
+
+    @Column({ type: "varchar", nullable: true })
+    author: string | null
+}

test/github-issues/2331/issue-2331.ts

@@ -0,0 +1,59 @@
+import "reflect-metadata"
+import { expect } from "chai"
+import { DataSource, Repository } from "../../../src"
+import { Post } from "./entity/Post"
+import {
+    reloadTestingDatabases,
+    closeTestingConnections,
+    setupSingleTestingConnection,
+} from "../../utils/test-utils"
+
+describe("github issues > #2331 undefined value is nulling column on update", () => {
+    let dataSource: DataSource
+    let repository: Repository<Post>
+
+    before(async () => {
+        const options = setupSingleTestingConnection("postgres", {
+            entities: [__dirname + "/entity/*{.js,.ts}"],
+            schemaCreate: true,
+            dropSchema: true,
+        })
+
+        if (!options) return
+
+        dataSource = new DataSource(options)
+        await dataSource.initialize()
+    })
+    beforeEach(async () => {
+        if (!dataSource) return
+        await reloadTestingDatabases([dataSource])
+        repository = dataSource.getRepository(Post)
+    })
+    after(() => closeTestingConnections([dataSource]))
+
+    it("should not overwrite column with null when passing undefined", async () => {
+        if (!dataSource) return
+
+        const post = new Post()
+        post.id = 1
+        post.title = "Some post"
+        post.author = "Some author"
+
+        await repository.save(post)
+        await repository.update(
+            {
+                id: post.id,
+            },
+            {
+                title: "Updated post",
+                author: undefined,
+            },
+        )
+        const postReloaded = await repository.findOne({
+            where: { id: post.id },
+        })
+
+        expect(postReloaded).to.exist
+        expect(postReloaded!.author).to.be.equal("Some author")
+    })
+})