This post is something I find myself needing a lot, so I thought I would put it here for my own memory and anyone else who might need it. To give credit where credit is due, the methodology shown below is taken from the answers here: https://laracasts.com/discuss/channels/requests/many-to-many-relation-with-unique-records-in-pivot?page=1
Often times, when I set up a many-to-many relationship in Laravel, I need the relationship between two nodes to be distinct. For example, when a user is given a privilege for a forum, and they can have one-and-only-one privilege type for that particular forum, even if they are related to many other forums, then we will not want duplicate pivot rows. In this case, it would get awful tedious to ensure that every time there is an update for the user’s privilege that we check to see if a relationship already exists between the user and forum and update it instead of creating it. That can be thrown into a model method easily enough for DRY’er code, but it is still mildly annoying, and if you’re working with a team there’s no ultimate failsafe error should someone circumvent that procedure, by accident or otherwise.
So the goal of this post is:
- to ensure that in a many-to-many relationship there can be only one row relating two nodes (many users are tied to many forums, but a particular user and a particular forum are related through no more than one row);
- to use built-in Eloquent methodology to perform the if-new-create-otherwise-update logic for us; and
- to ensure that the systems used will throw an error should an attempt be made to create a duplicate relationship.
Most of the heavy lifting for this setup will actually be accomplished by our database.
The first thing we need to look at is how we set up the pivot table for our relationship. Ordinarily a pivot table definition might look something like this in a migration file:
The problem with this setup is that there is nothing stopping many rows with the same user_id and forum_id from being created. To remedy this, we’ll make the combination of the user_id and forum_id our primary key, which allows us to remove the id column entirely.
The resulting pivot table migration will look something like this:
Of our original 3 goals, 1 and 3 have now been satisfied. For our first goal, the database will accept one-and-only-one row that relates a specific user to a specific forum. If a second row with the same combination of user and forum is given to the database for creation an error will be thrown, which will help save us from ourselves in the future if we forget the uniqueness of this relationship.
So that just leaves the second goal. Thankfully this is easily satisfied using Eloquent’s sync method.
Normally we pass a list of ids to sync, and Eloquent will ensure that the model being sync’ed is related to the given ids of the other model and only those ids. So this method will create any missing relations and remove any extra relations. This gets us halfway there.
The problem is that sync is destructive when we don’t want it to be. The second parameter for sync allows us to turn off the destructive behavior. By passing false along with our array of ids, sync will create any missing relationships, update existing ones as necessary, and disregard the extra ones.
The result will look something like this:
That will do it, all 3 of the original goals have now been satisfied.
As a bonus, if you have extra columns of data in your pivot table, like the access_level column we defined above, you can use sync and pass that data as well. The general idea is that you construct an array where the keys are the ids, and the values are associative arrays of data to save to the pivot table.
Here is what that might look like: