diff --git a/app/Models/Assignment.php b/app/Models/Assignment.php
index a94d5cb..9e887c2 100644
--- a/app/Models/Assignment.php
+++ b/app/Models/Assignment.php
@@ -16,14 +16,7 @@ class Assignment extends Model implements HasMedia
 {
     use HasFactory, InteractsWithMedia, SoftDeletes;
 
-    protected $fillable = [
-        'course_id',
-        'class_session_id',
-        'description',
-        'due_date',
-        'title',
-        'type',
-    ];
+    protected $guarded = ['id'];
 
     protected function casts(): array
     {
@@ -48,16 +41,16 @@ public function assignmentTargets(): HasMany
         return $this->hasMany(AssignmentTarget::class);
     }
 
-    public function course(): BelongsTo
-    {
-        return $this->belongsTo(Course::class);
-    }
-
     public function classSession(): BelongsTo
     {
         return $this->belongsTo(ClassSession::class);
     }
 
+    public function course(): BelongsTo
+    {
+        return $this->belongsTo(Course::class);
+    }
+
     public function students(): BelongsToMany
     {
         return $this->belongsToMany(
diff --git a/app/Models/AssignmentPin.php b/app/Models/AssignmentPin.php
index 9349c9f..884e27b 100644
--- a/app/Models/AssignmentPin.php
+++ b/app/Models/AssignmentPin.php
@@ -7,10 +7,7 @@
 
 class AssignmentPin extends Model
 {
-    protected $fillable = [
-        'assignment_id',
-        'student_id',
-    ];
+    protected $guarded = ['id'];
 
     public function assignment(): BelongsTo
     {
diff --git a/app/Models/AssignmentSubmission.php b/app/Models/AssignmentSubmission.php
index fd4842d..d927fb9 100644
--- a/app/Models/AssignmentSubmission.php
+++ b/app/Models/AssignmentSubmission.php
@@ -12,13 +12,7 @@ class AssignmentSubmission extends Model implements HasMedia
 {
     use HasFactory, InteractsWithMedia;
 
-    protected $fillable = [
-        'assignment_id',
-        'notes',
-        'student_id',
-        'study_group_id',
-        'submitted_at',
-    ];
+    protected $guarded = ['id'];
 
     protected function casts(): array
     {
diff --git a/app/Models/AssignmentTarget.php b/app/Models/AssignmentTarget.php
index 9717c29..e4f5cac 100644
--- a/app/Models/AssignmentTarget.php
+++ b/app/Models/AssignmentTarget.php
@@ -10,11 +10,7 @@ class AssignmentTarget extends Model
 {
     use HasFactory;
 
-    protected $fillable = [
-        'assignment_id',
-        'student_id',
-        'study_group_id',
-    ];
+    protected $guarded = ['id'];
 
     public function assignment(): BelongsTo
     {
diff --git a/app/Models/Attendance.php b/app/Models/Attendance.php
index 4eefa57..0d72e00 100644
--- a/app/Models/Attendance.php
+++ b/app/Models/Attendance.php
@@ -3,6 +3,7 @@
 namespace App\Models;
 
 use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\BelongsTo;
 
 class Attendance extends Model
 {
@@ -16,18 +17,18 @@ protected function casts(): array
         ];
     }
 
-    public function student()
+    public function classSession(): BelongsTo
     {
-        return $this->belongsTo(Student::class);
+        return $this->belongsTo(ClassSession::class);
     }
 
-    public function courseSchedule()
+    public function courseSchedule(): BelongsTo
     {
         return $this->belongsTo(CourseSchedule::class);
     }
 
-    public function classSession()
+    public function student(): BelongsTo
     {
-        return $this->belongsTo(ClassSession::class);
+        return $this->belongsTo(Student::class);
     }
 }
diff --git a/app/Models/ClassSession.php b/app/Models/ClassSession.php
index 44ccde8..d07740a 100644
--- a/app/Models/ClassSession.php
+++ b/app/Models/ClassSession.php
@@ -12,29 +12,16 @@ class ClassSession extends Model
 {
     use HasFactory, SoftDeletes;
 
-    protected $fillable = [
-        'course_id',
-        'session_number',
-        'date',
-        'start_time',
-        'end_time',
-    ];
+    protected $guarded = ['id'];
 
-    protected $casts = [
-        'date' => 'date',
-        'session_number' => 'integer',
-        'start_time' => 'datetime',
-        'end_time' => 'datetime',
-    ];
-
-    public function course(): BelongsTo
+    protected function casts(): array
     {
-        return $this->belongsTo(Course::class);
-    }
-
-    public function materials(): HasMany
-    {
-        return $this->hasMany(Material::class);
+        return [
+            'date' => 'date',
+            'session_number' => 'integer',
+            'start_time' => 'datetime',
+            'end_time' => 'datetime',
+        ];
     }
 
     public function assignments(): HasMany
@@ -46,4 +33,14 @@ public function attendances(): HasMany
     {
         return $this->hasMany(Attendance::class);
     }
+
+    public function course(): BelongsTo
+    {
+        return $this->belongsTo(Course::class);
+    }
+
+    public function materials(): HasMany
+    {
+        return $this->hasMany(Material::class);
+    }
 }
diff --git a/app/Models/Course.php b/app/Models/Course.php
index 9d55ebf..eb9492f 100644
--- a/app/Models/Course.php
+++ b/app/Models/Course.php
@@ -21,6 +21,21 @@ protected static function booted()
         });
     }
 
+    public function assignments(): HasMany
+    {
+        return $this->hasMany(Assignment::class);
+    }
+
+    public function classSessions(): HasMany
+    {
+        return $this->hasMany(ClassSession::class);
+    }
+
+    public function courseSchedules(): HasMany
+    {
+        return $this->hasMany(CourseSchedule::class);
+    }
+
     public function materials(): HasMany
     {
         return $this->hasMany(Material::class);
@@ -30,14 +45,4 @@ public function studyGroups(): BelongsToMany
     {
         return $this->belongsToMany(StudyGroup::class, 'study_group_courses');
     }
-
-    public function courseSchedules(): HasMany
-    {
-        return $this->hasMany(CourseSchedule::class);
-    }
-
-    public function classSessions(): HasMany
-    {
-        return $this->hasMany(ClassSession::class);
-    }
 }
diff --git a/app/Models/Student.php b/app/Models/Student.php
index 052c4b6..bee66a6 100644
--- a/app/Models/Student.php
+++ b/app/Models/Student.php
@@ -8,6 +8,7 @@
 use Illuminate\Database\Eloquent\Factories\HasFactory;
 use Illuminate\Database\Eloquent\Model;
 use Illuminate\Database\Eloquent\Relations\BelongsTo;
+use Illuminate\Database\Eloquent\Relations\BelongsToMany;
 use Illuminate\Database\Eloquent\Relations\HasMany;
 use Illuminate\Database\Eloquent\SoftDeletes;
 
@@ -37,13 +38,53 @@ protected function female(Builder $query): void
         $query->where('sex', Sex::Female);
     }
 
-    public function user(): BelongsTo
+    public function assignmentPins(): HasMany
     {
-        return $this->belongsTo(User::class);
+        return $this->hasMany(AssignmentPin::class);
+    }
+
+    public function assignments(): BelongsToMany
+    {
+        return $this->belongsToMany(
+            Assignment::class,
+            'assignment_targets',
+            'student_id',
+            'assignment_id'
+        );
+    }
+
+    public function assignmentSubmissions(): HasMany
+    {
+        return $this->hasMany(AssignmentSubmission::class);
+    }
+
+    public function assignmentTargets(): HasMany
+    {
+        return $this->hasMany(AssignmentTarget::class);
     }
 
     public function attendances(): HasMany
     {
         return $this->hasMany(Attendance::class);
     }
+
+    public function ledStudyGroups(): HasMany
+    {
+        return $this->hasMany(StudyGroup::class, 'leader_id');
+    }
+
+    public function studyGroupMembers(): HasMany
+    {
+        return $this->hasMany(StudyGroupMember::class);
+    }
+
+    public function studyGroups(): BelongsToMany
+    {
+        return $this->belongsToMany(StudyGroup::class, 'study_group_members');
+    }
+
+    public function user(): BelongsTo
+    {
+        return $this->belongsTo(User::class);
+    }
 }
diff --git a/app/Models/StudyGroup.php b/app/Models/StudyGroup.php
index 24944bf..d829094 100644
--- a/app/Models/StudyGroup.php
+++ b/app/Models/StudyGroup.php
@@ -13,16 +13,33 @@ class StudyGroup extends Model
 {
     use HasFactory, SoftDeletes;
 
-    protected $fillable = [
-        'leader_id',
-        'name',
-    ];
+    protected $guarded = ['id'];
 
     public function isLeader(Student $student): bool
     {
         return $this->leader_id === $student->id;
     }
 
+    public function assignmentSubmissions(): HasMany
+    {
+        return $this->hasMany(AssignmentSubmission::class);
+    }
+
+    public function assignmentTargets(): HasMany
+    {
+        return $this->hasMany(AssignmentTarget::class);
+    }
+
+    public function assignments(): BelongsToMany
+    {
+        return $this->belongsToMany(
+            Assignment::class,
+            'assignment_targets',
+            'study_group_id',
+            'assignment_id'
+        );
+    }
+
     public function courses(): BelongsToMany
     {
         return $this->belongsToMany(Course::class, 'study_group_courses');
diff --git a/app/Models/StudyGroupMember.php b/app/Models/StudyGroupMember.php
index ff25117..f88e759 100644
--- a/app/Models/StudyGroupMember.php
+++ b/app/Models/StudyGroupMember.php
@@ -10,10 +10,7 @@ class StudyGroupMember extends Model
 {
     use HasFactory;
 
-    protected $fillable = [
-        'student_id',
-        'study_group_id',
-    ];
+    protected $guarded = ['id'];
 
     public function student(): BelongsTo
     {