[10.x] Allow for converting a HasMany to HasOne && MorphMany to MorphOne

src/Illuminate/Database/Eloquent/Relations/HasMany.php

@@ -6,6 +6,16 @@
 
 class HasMany extends HasOneOrMany
 {
+    /**
+     * Convert the relationship to a "has one" relationship.
+     *
+     * @return Illuminate\Database\Eloquent\Relations\HasOne
+     */
+    public function one()
+    {
+        return new HasOne($this->getQuery(), $this->parent, $this->foreignKey, $this->localKey);
+    }
+
     /**
      * Get the results of the relationship.
      *

src/Illuminate/Database/Eloquent/Relations/HasManyThrough.php

@@ -81,6 +81,24 @@ public function __construct(Builder $query, Model $farParent, Model $throughPare
         parent::__construct($query, $throughParent);
     }
 
+    /**
+     * Convert the relationship to a "has one through" relationship.
+     *
+     * @return \Illuminate\Database\Eloquent\Relations\HasOneThrough
+     */
+    public function one()
+    {
+        return new HasOneThrough(
+            $this->getQuery(),
+            $this->farParent,
+            $this->throughParent,
+            $this->getFirstKeyName(),
+            $this->secondKey,
+            $this->getLocalKeyName(),
+            $this->getSecondLocalKeyName(),
+        );
+    }
+
     /**
      * Set the base constraints on the relation query.
      *

src/Illuminate/Database/Eloquent/Relations/MorphMany.php

@@ -6,6 +6,16 @@
 
 class MorphMany extends MorphOneOrMany
 {
+    /**
+     * Convert the relationship to a "morph one" relationship.
+     *
+     * @return \Illuminate\Database\Eloquent\Relations\MorphOne
+     */
+    public function one()
+    {
+        return new MorphOne($this->getQuery(), $this->getParent(), $this->morphType, $this->foreignKey, $this->localKey);
+    }
+
     /**
      * Get the results of the relationship.
      *

tests/Integration/Database/EloquentHasManyTest.php

@@ -0,0 +1,81 @@
+<?php
+
+namespace Illuminate\Tests\Integration\Database;
+
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\HasMany;
+use Illuminate\Database\Eloquent\Relations\HasOne;
+use Illuminate\Support\Facades\Schema;
+
+class EloquentHasManyTest extends DatabaseTestCase
+{
+    protected function defineDatabaseMigrationsAfterDatabaseRefreshed()
+    {
+        Schema::create('eloquent_has_many_test_users', function ($table) {
+            $table->id();
+        });
+
+        Schema::create('eloquent_has_many_test_logins', function ($table) {
+            $table->id();
+            $table->foreignId('eloquent_has_many_test_user_id');
+            $table->timestamp('login_time');
+        });
+    }
+
+    public function testCanGetHasOneFromHasManyRelationship()
+    {
+        $user = EloquentHasManyTestUser::create();
+
+        $user->logins()->create(['login_time' => now()]);
+
+        $this->assertInstanceOf(HasOne::class, $user->logins()->one());
+    }
+
+    public function testHasOneRelationshipFromHasMany()
+    {
+        $user = EloquentHasManyTestUser::create();
+
+        EloquentHasManyTestLogin::create([
+            'eloquent_has_many_test_user_id' => $user->id,
+            'login_time' => '2020-09-29',
+        ]);
+        $latestLogin = EloquentHasManyTestLogin::create([
+            'eloquent_has_many_test_user_id' => $user->id,
+            'login_time' => '2023-03-14',
+        ]);
+        $oldestLogin = EloquentHasManyTestLogin::create([
+            'eloquent_has_many_test_user_id' => $user->id,
+            'login_time' => '2010-01-01',
+        ]);
+
+        $this->assertEquals($oldestLogin->id, $user->oldestLogin->id);
+        $this->assertEquals($latestLogin->id, $user->latestLogin->id);
+    }
+}
+
+class EloquentHasManyTestUser extends Model
+{
+    protected $guarded = [];
+    public $timestamps = false;
+
+    public function logins(): HasMany
+    {
+        return $this->hasMany(EloquentHasManyTestLogin::class);
+    }
+
+    public function latestLogin(): HasOne
+    {
+        return $this->logins()->one()->latestOfMany('login_time');
+    }
+
+    public function oldestLogin(): HasOne
+    {
+        return $this->logins()->one()->oldestOfMany('login_time');
+    }
+}
+
+class EloquentHasManyTestLogin extends Model
+{
+    protected $guarded = [];
+    public $timestamps = false;
+}

tests/Integration/Database/EloquentMorphManyTest.php

@@ -3,7 +3,9 @@
 namespace Illuminate\Tests\Integration\Database\EloquentMorphManyTest;
 
 use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\MorphOne;
 use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Carbon;
 use Illuminate\Support\Facades\Schema;
 use Illuminate\Support\Str;
 use Illuminate\Tests\Integration\Database\DatabaseTestCase;
@@ -48,6 +50,25 @@ public function test_self_referencing_existence_query()
 
         $this->assertEquals([1], $comments->pluck('id')->all());
     }
+
+    public function testCanMorphOne()
+    {
+        $post = Post::create(['title' => 'Your favorite book by C.S. Lewis']);
+
+        Carbon::setTestNow('1990-02-02 12:00:00');
+        $oldestComment = tap((new Comment(['name' => 'The Allegory Of Love']))->commentable()->associate($post))->save();
+
+        Carbon::setTestNow('2000-07-02 09:00:00');
+        tap((new Comment(['name' => 'The Screwtape Letters']))->commentable()->associate($post))->save();
+
+        Carbon::setTestNow('2022-01-01 00:00:00');
+        $latestComment = tap((new Comment(['name' => 'The Silver Chair']))->commentable()->associate($post))->save();
+
+        $this->assertInstanceOf(MorphOne::class, $post->comments()->one());
+
+        $this->assertEquals($latestComment->id, $post->latestComment->id);
+        $this->assertEquals($oldestComment->id, $post->oldestComment->id);
+    }
 }
 
 class Post extends Model
@@ -61,6 +82,16 @@ public function comments()
     {
         return $this->morphMany(Comment::class, 'commentable');
     }
+
+    public function latestComment(): MorphOne
+    {
+        return $this->comments()->one()->latestOfMany();
+    }
+
+    public function oldestComment(): MorphOne
+    {
+        return $this->comments()->one()->oldestOfMany();
+    }
 }
 
 class Comment extends Model