Laravel Relationship methods: when to use associate(), sync(), and more

Laravel provides several handy affordances for working with relationships. Because there is almost at least one affordance method for a relationship, the sheer number of these affordances, while being helpful in their numbers, can make it confusing to master which to use, especially for beginners, or evening needing to skim the docs to get reminded. Should you use associate() or attach()? When is sync() the right choice?

In this article, we will discuss how to easily choose between relationship creation methods by asking the simple question: Is there an intermediate table involved? Let's break down these methods and their ideal use cases, focusing on a simple rule:

• Use associate() and dissociate() when dealing with one-to-one or belongs-to relationships.
• Use attach(), detach(), and sync() when dealing with many-to-many relationships that involve an intermediate table.

One-to-One and Belongs-To Relationships: associate() and dissociate()

When working with relationships where a child model belongs to a parent, such as a User belonging to an Account, you should use associate() and dissociate() to manage these relationships.

Example: Assigning a User to an Account

use App\Models\User;
use App\Models\Account;

$user = User::find(1);
$account = Account::find(10);

// Assign the user to an account
$user->account()->associate($account);
$user->save();

Here, associate() updates the foreign key on the users table to reference the given account.

Example: Removing a User’s Account Association

$user->account()->dissociate();
$user->save();

The dissociate() method sets the foreign key to NULL, effectively removing the relationship.

When to Use These Methods

• Use associate() when linking a child model to a new parent.
• Use dissociate() when unassigning the parent model.
• No intermediate table is involved—only a direct foreign key reference.

Many-to-Many Relationships: attach(), detach(), and sync()

When dealing with many-to-many relationships (e.g., a relationship between User and Role needs a role_user table), Laravel uses an intermediate table. In these cases, you should use methods like attach(), detach(), and sync() to manage associations.

Example: Attaching a Role to a User

use App\Models\User;

$user = User::find(1);
$roleId = 3;

// Attach role to user
$user->roles()->attach($roleId);

This adds a new row in the role_user pivot table, linking the user and role.

Example: Attaching a Role with Additional Data

$user->roles()->attach($roleId, ['expires' => now()->addMonth()]);

Here, we store extra metadata (e.g., an expiration date) in the pivot table.

Example: Detaching a Role

$user->roles()->detach($roleId);

The detach() method removes the specific record from the pivot table.

Example: Syncing Roles

If you want to update a user’s roles so that only specific ones remain, use sync():

$user->roles()->sync([1, 2, 3]);

This ensures that only roles 1, 2, and 3 remain attached; any others are removed.

If you need to preserve existing roles and just add new ones, use syncWithoutDetaching():

$user->roles()->syncWithoutDetaching([4, 5]);

This keeps all existing roles and only adds roles 4 and 5 if they are not already attached.

When to Use These Methods

• Use attach() to add a record to the pivot table.
• Use detach() to remove a record from the pivot table.
• Use sync() to set an exact list of relationships, removing any that aren't in the list.
• Use syncWithoutDetaching() to add new relationships while keeping existing ones.
• Intermediate table is required (e.g., role_user for User ↔ Role).

Conclusion

To choose the right method, remember this simple distinction:

• Use associate() and dissociate() for one-to-one or belongs-to relationships where a foreign key exists in the child table.
• Use attach(), detach(), and sync() for many-to-many relationships involving an intermediate table.

Hopefully, you find it easier to know when to use each of the Eloquent relationship methods discussed above. See you next time!