fix: corrects local strategy user lookup when using loginWithUsername (#7587)

## Description

Fixes the local strategy user lookup.

- [x] I have read and understand the
[CONTRIBUTING.md](https://github.com/payloadcms/payload/blob/main/CONTRIBUTING.md)
document in this repository.

## Type of change

- [x] Bug fix (non-breaking change which fixes an issue)

## Checklist:

- [x] I have added tests that prove my fix is effective or that my
feature works
- [ ] Existing test suite passes locally with my changes
- [ ] I have made corresponding changes to the documentation

packages/payload/src/auth/strategies/local/register.ts

@@ -1,6 +1,6 @@
 import type { SanitizedCollectionConfig } from '../../../collections/config/types.js'
 import type { JsonObject, Payload } from '../../../index.js'
-import type { PayloadRequest } from '../../../types/index.js'
+import type { PayloadRequest, Where } from '../../../types/index.js'
 
 import { ValidationError } from '../../../errors/index.js'
 import { generatePasswordSaltHash } from './generatePasswordSaltHash.js'
@@ -22,23 +22,43 @@ export const registerLocalStrategy = async ({
 }: Args): Promise<Record<string, unknown>> => {
   const loginWithUsername = collection?.auth?.loginWithUsername
 
+  let whereConstraint: Where
+
+  if (!loginWithUsername) {
+    whereConstraint = {
+      email: {
+        equals: doc.email,
+      },
+    }
+  } else {
+    whereConstraint = {
+      or: [],
+    }
+
+    if (doc.email) {
+      whereConstraint.or.push({
+        email: {
+          equals: doc.email,
+        },
+      })
+    }
+
+    if (doc.username) {
+      whereConstraint.or.push({
+        username: {
+          equals: doc.username,
+        },
+      })
+    }
+  }
+
   const existingUser = await payload.find({
     collection: collection.slug,
     depth: 0,
     limit: 1,
     pagination: false,
     req,
-    where: loginWithUsername
-      ? {
-          username: {
-            equals: doc.username,
-          },
-        }
-      : {
-          email: {
-            equals: doc.email,
-          },
-        },
+    where: whereConstraint,
   })
 
   if (existingUser.docs.length > 0) {

test/login-with-username/int.spec.ts

@@ -17,22 +17,20 @@ describe('Login With Username Feature', () => {
     }
   })
 
-  describe('hook execution', () => {
-    it('should not allow creation with neither email nor username', async () => {
-      let errors = []
-      try {
-        await payload.create({
-          collection: 'login-with-either',
-          data: {
-            email: null,
-            username: null,
-          },
-        })
-      } catch (error) {
-        errors = error.data.errors
-      }
-      expect(errors).toHaveLength(2)
-    })
+  it('should not allow creation with neither email nor username', async () => {
+    let errors = []
+    try {
+      await payload.create({
+        collection: 'login-with-either',
+        data: {
+          email: null,
+          username: null,
+        },
+      })
+    } catch (error) {
+      errors = error.data.errors
+    }
+    expect(errors).toHaveLength(2)
   })
 
   it('should not allow removing both username and email fields', async () => {
@@ -115,4 +113,44 @@ describe('Login With Username Feature', () => {
     })
     expect(loginWithUsername).toHaveProperty('token')
   })
+
+  it('should allow mutliple creates with optional email and username', async () => {
+    // create a user with just email
+    await payload.create({
+      collection: 'login-with-either',
+      data: {
+        email: 'email1@mail.com',
+        password: 'test',
+      },
+    })
+
+    // create second user with just email
+    const emailUser2 = await payload.create({
+      collection: 'login-with-either',
+      data: {
+        email: 'email2@mail.com',
+        password: 'test',
+      },
+    })
+    expect(emailUser2).toHaveProperty('id')
+
+    // create user with just username
+    await payload.create({
+      collection: 'login-with-either',
+      data: {
+        username: 'username1',
+        password: 'test',
+      },
+    })
+
+    // create second user with just username
+    const usernameUser2 = await payload.create({
+      collection: 'login-with-either',
+      data: {
+        username: 'username2',
+        password: 'test',
+      },
+    })
+    expect(usernameUser2).toHaveProperty('id')
+  })
 })