Laravel save / update many to many relationship

tldr; Use sync with 2nd param false

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Many-to-many relationship is belongsToMany on both models:

// Task model
public function users()
{
  return $this->belongsToMany('User', 'user_tasks'); // assuming user_id and task_id as fk
}

// User model
public function tasks()
{
  return $this->belongsToMany('Task', 'user_tasks');
}

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In order to add new relation use attach or sync.

Difference between the two is:

1 attach will add new row on the pivot table without checking if it's already there. It's good when you have additional data linked to that relation, for example:

User and Exam linked with pivot table attempts: id, user_id, exam_id, score

I suppose this is not what you need in your situation:

$user->tasks()->getRelatedIds(); // [1,2,3,4,5,6]

$user->tasks()->attach([5,6,7]);
// then
$user->tasks()->getRelatedIds(); // [1,2,3,4,5,6,5,6,7]

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2 sync on the other hand, will either remove all relations and set them up anew:

$user->tasks()->getRelatedIds(); // [1,2,3,4,5,6]

$user->tasks()->sync([1,2,3]);
// then
$user->tasks()->getRelatedIds(); // [1,2,3]

or it will setup new relations without detaching previous AND without adding duplicates:

$user->tasks()->sync([5,6,7,8], false); // 2nd param = detach
// then
$user->tasks()->getRelatedIds(); // [1,2,3,4,5,6,7,8]

Here's my notes on how to save and update on all the Eloquent relationships.

in One to One:

You have to use HasOne on the first model and BelongsTo on the second model

to add record on the first model (HasOne) use the save function

example:    $post->comments()->save($comment);

to add record on the second model (BelongsTo) use the associate function

example:    $user->account()->associate($account);    $user->save();

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in One to Many:

You have to use HasMany on the first model and BelongsTo on the second model

to add record on the first table (HasMany) use the save or saveMany functions

example:    $post->comments()->saveMany($comments);

to add record on the second model (BelongsTo) use the associate function

example:    $user->account()->associate($account);    $user->save();

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in Many to Many:

You have to use BelongsToMany on the first model and BelongsToMany on the second model

to add records on the pivot table use attach or sync functions

• both functions accepts single ID or array of ID’s 

• the difference is attach checks if the record already exist on the pivot table while sync don’t

example: $user->roles()->attach($roleId);

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in Polymorphic One to Many:

You have to use MorphMany on the main model and MorphTo on all the (***able) models

to add records on all the other models use the save

example:    $course->tags()->save($tag);

the pivot table should have the following columns:

. main model ID

. (***able) ID

. (***able) Type

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in Polymorphic Many to Many:

You have to use MorphByMany on the main model and MorphToMany on all the (***able) models

to add records on all the other models use the save or saveMany

example:    $course->tags()->save($tag);

example:    $course->tags()->saveMany([$tag_1, $tag_2, $tag_3]);

the pivot table should have the following columns:

. main model ID

. (***able) ID

. (***able) Type

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in Has Many Through (shortcut):

You have to use HasManyThrough on the first table and have the normal relations on the other 2 tables

this doesn’t work for ManyToMany relationships (where there’s a pivot table)

however there’s a nice and easy solution just for that.

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Here's an article I wrote, inspired by this answer. Important to check it: https://hackernoon.com/eloquent-relationships-cheat-sheet-5155498c209