In Laravel, Eloquent is a cornerstone for interacting with databases. Among its many features, pivot tables stand out for managing many-to-many relationships. These tables serve as a bridge, linking different models in a many-to-many relationship, such as users and projects. However, there comes a point in application development where one must reevaluate the role and structure of these pivot tables.

Invisible Tables, Visible Signs

At first, a pivot table doesn't need a model at all. Say you've got a User model and a Project model. Laravel connects them through a project_user table with user_id and project_id columns, and you never have to think about it:

php
class User extends Model
{
    public function projects()
    {
        return $this->belongsToMany(Project::class);
    }
}

class Project extends Model
{
    public function users()
    {
        return $this->belongsToMany(User::class);
    }
}

That works great until you want to hang behavior or extra data onto the relationship itself.

The Smell of Change

Let's say you want to track when a user joined a project, or why they were added. You can add joined_at and reason columns to project_user and read them with withPivot():

php
public function projects()
{
    return $this->belongsToMany(Project::class)->withPivot('joined_at', 'reason');
}

That's fine for a column or two. But once you're reaching for $project->pivot->reason all over the place, or the relationship needs its own methods and rules, the pivot table is doing a model's job.

Give it a model

When the pivot table has its own fields and rules, I'd give it a model. For our example, that's ProjectMembership:

php
class ProjectMembership extends Model
{
    protected $table = 'project_user';

    protected $fillable = ['user_id', 'project_id', 'joined_at', 'reason'];

    public function user()
    {
        return $this->belongsTo(User::class);
    }

    public function project()
    {
        return $this->belongsTo(Project::class);
    }
}

Note: Without the $table property, Eloquent will look for a project_memberships table. Either set it, or rename the table in a migration.

Now the membership is a real thing in your app, and you can give it methods, scopes and validation like any other model.

Updating the relationships

With the new model in place, User and Project each have many memberships:

php
class User extends Model
{
    public function projectMemberships()
    {
        return $this->hasMany(ProjectMembership::class);
    }
}

class Project extends Model
{
    public function projectMemberships()
    {
        return $this->hasMany(ProjectMembership::class);
    }
}

If you'd rather keep $user->projects working too, Laravel also supports custom pivot models. Extend Pivot instead of Model, and point the relationship at it with using().

A column or two on the pivot table is fine. Once the relationship needs its own methods or rules, give it a model.