Better way to find unique record in has_many through relationship

Question

In my rails app, a user can go along to events -

user.rb

has_many :attendances
has_many :events, through: :attendances

event.rb

has_many :attendances
has_many :users, through: :attendances

... with an attendance table which is made up of event_id, user_id and some other bits and pieces -

attendance.rb

belongs_to :user
belongs_to :writers_event

When looking for a particular attendance I find myself using .where ... .first - e.g.

attendance = Attendance.where(user_id: @user.id, event_id: this_event_id).first

And it strikes me that I missed the class where we talked about using something like find_by in this type of situation - in other words, where you are confident that you are looking for something unique. It might not matter, but searching for a collection then taking the first object from it feels wasteful and wrong.

Is there a better way?

I had a look around, and this was closest, but does not (I don't think) really cover it. How to display unique records from a has_many through relationship?

score 5 · Answer 1 · answered Nov 20 '15 at 11:05

5

It's actually pretty straightforward:

attendance = Attendance.find_by(user_id: @user.id, event_id: this_event_id)

You just pass the same conditions to find_by, and it returns you the first record.

There is one catch though. By using find_by, you will get nil, if nothing is found. If you need to get an ActiveRecord::RecordNotFound raised, if nothing is found, you can use find_by!.

answered Nov 20 '15 at 11:05

Yury Lebedev

3,985
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The find_by behaviour regarding nils and exceptions is very important to remember, as it can bite. – A Fader Darkly Nov 20 '15 at 12:09

score 3 · Accepted Answer · answered Nov 20 '15 at 11:07

3

You can use find_by for this:

attendance = Attendance.find_by(user_id: @user.id, event_id: this_event_id)

Although behind the scenes, it is doing a look-up and taking the first.

However, you can get a bit more help from rails here:

current_user.attendances.find_by(event_id: this_event_id)

answered Nov 20 '15 at 11:07

A Fader Darkly

3,516
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1

Obviously you can do the same for the event: ```my_cool_event.attendances.find_by(user_id: @user.id)``` – A Fader Darkly Nov 20 '15 at 11:09

score 2 · Answer 3 · answered Nov 20 '15 at 11:14

2

Try this one:

Attendance.find_by_user_and_event(@user.id, this_event_id)

If the record not found then it return nil. If you want to raise the exception

Attendance.find_by_user_and_event!(@user.id, this_event_id)

answered Nov 20 '15 at 11:14

Kh Ammad

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According to Upgrading to Rails 4.0 `All dynamic methods except for find_by_... and find_by_...! are deprecated` – Kh Ammad Nov 20 '15 at 11:30
Vote up, if you like my answer ;) – Kh Ammad Nov 20 '15 at 11:41

Better way to find unique record in has_many through relationship

3 Answers3