fix: use an empty string enum as the type of a primary key column

src/query-builder/transformer/RawSqlResultsToEntityTransformer.ts

@@ -72,7 +72,6 @@ export class RawSqlResultsToEntityTransformer {
 
                 return keyValue;
             }).join("_"); // todo: check partial
-            if (!id) return;
 
             const items = map.get(id);
             if (!items) {

test/github-issues/3874/entity/Settings.ts

@@ -0,0 +1,16 @@
+import {Entity, Column, PrimaryColumn} from "../../../../src";
+
+enum Singleton {
+    EMPTY = ""
+}
+
+@Entity()
+export class Settings {
+
+    @PrimaryColumn()
+    readonly singleton: Singleton = Singleton.EMPTY;
+
+    @Column()
+    value!: string;
+
+}

test/github-issues/3874/issue-3874.ts

@@ -0,0 +1,30 @@
+import "reflect-metadata";
+import {createTestingConnections, closeTestingConnections, reloadTestingDatabases} from "../../utils/test-utils";
+import {Connection} from "../../../src/connection/Connection";
+import {Settings} from "./entity/Settings";
+import {expect} from "chai";
+
+describe("github issues > #3874 Using an (empty string) enum as the type of a primary key column", () => {
+
+    let connections: Connection[];
+    before(async () => connections = await createTestingConnections({
+        entities: [Settings],
+        enabledDrivers: ["mysql", "mariadb"],
+        schemaCreate: true,
+        dropSchema: true,
+    }));
+    beforeEach(() => reloadTestingDatabases(connections));
+    after(() => closeTestingConnections(connections));
+
+    it("should reload entity", () => Promise.all(connections.map(async connection => {
+        // Create initial settings row
+        const newSettings = new Settings();
+        newSettings.value = "string";
+        await connection.manager.save(newSettings);
+        // Attempt to read settings back
+        const foundSettings = await connection.manager.findOne(Settings);
+        expect(foundSettings).to.be.an.instanceOf(Settings);
+        expect(foundSettings != null ? foundSettings.value : null).to.equal("string");
+    })));
+
+});