fix: prevent modification of the FindOptions.relations (#7887)

because of how we modify it as part of a recursive function
we were erasing the `relations` array when iterating over them.
this fixes that so we don't mess with the inputs like that
and instead copy it

src/find-options/FindOptionsUtils.ts

@@ -111,7 +111,8 @@ export class FindOptionsUtils {
         }
 
         if (options.relations) {
-            const allRelations = options.relations;
+            // Copy because `applyRelationsRecursively` modifies it
+            const allRelations = [...options.relations];
             this.applyRelationsRecursively(qb, allRelations, qb.expressionMap.mainAlias!.name, qb.expressionMap.mainAlias!.metadata, "");
             // recursive removes found relations from allRelations array
             // if there are relations left in this array it means those relations were not found in the entity structure

test/github-issues/7882/entity/Example.ts

@@ -0,0 +1,12 @@
+import { Entity, JoinColumn, OneToOne, PrimaryGeneratedColumn } from "../../../../src";
+import { ExampleText } from "./ExampleText";
+
+@Entity()
+export class Example {
+    @PrimaryGeneratedColumn()
+    id: string;
+
+    @OneToOne(() => ExampleText)
+    @JoinColumn()
+    exampleText: ExampleText;
+}

test/github-issues/7882/entity/ExampleText.ts

@@ -0,0 +1,7 @@
+import { Entity, PrimaryGeneratedColumn } from "../../../../src";
+
+@Entity()
+export class ExampleText {
+    @PrimaryGeneratedColumn()
+    id: string;
+}

test/github-issues/7882/issue-7882.ts

@@ -0,0 +1,31 @@
+import "reflect-metadata";
+import { expect } from "chai";
+import { Connection } from "../../../src";
+import { closeTestingConnections, createTestingConnections, reloadTestingDatabases } from "../../utils/test-utils";
+import { Example } from "./entity/Example";
+import { ExampleText } from "./entity/ExampleText";
+
+describe.only("github issues > #7882  .findOne reduces relations to an empty array", () => {
+
+    let connections: Connection[];
+    before(async () => {
+        connections = await createTestingConnections({
+            enabledDrivers: [ "sqlite" ],
+            entities: [ Example, ExampleText ],
+            schemaCreate: false,
+            dropSchema: true
+        });
+    });
+    beforeEach(() => reloadTestingDatabases(connections));
+    after(() => closeTestingConnections(connections));
+
+    it("should delete all documents related to search pattern", () => Promise.all(connections.map(async connection => {
+        const relations = [ "exampleText" ];
+
+        const repo = connection.getRepository(Example);
+
+        await repo.find({ relations });
+
+        expect(relations).to.be.eql([ "exampleText" ]);
+    })));
+});