Rails find_or_create_by more than one attribute?

Question

There is a handy dynamic attribute in active-record called find_or_create_by:

Model.find_or_create_by_<attribute>(:<attribute> => "")

But what if I need to find_or_create by more than one attribute?

Say I have a model to handle a M:M relationship between Group and Member called GroupMember. I could have many instances where member_id = 4, but I don't ever want more than once instance where member_id = 4 and group_id = 7. I'm trying to figure out if it's possible to do something like this:

GroupMember.find_or_create(:member_id => 4, :group_id => 7)

I realize there may be better ways to handle this, but I like the convenience of the idea of find_or_create.

Ferrante

Answer 1

Multiple attributes can be connected with an and:

GroupMember.find_or_create_by_member_id_and_group_id(4, 7)

(use find_or_initialize_by if you don't want to save the record right away)

Edit: The above method is deprecated in Rails 4. The new way to do it will be:

GroupMember.where(:member_id => 4, :group_id => 7).first_or_create

and

GroupMember.where(:member_id => 4, :group_id => 7).first_or_initialize

Edit 2: Not all of these were factored out of rails just the attribute specific ones.

https://github.com/rails/rails/blob/4-2-stable/guides/source/active_record_querying.md

Example

GroupMember.find_or_create_by_member_id_and_group_id(4, 7)

became

GroupMember.find_or_create_by(member_id: 4, group_id: 7)

Racoon

You might also like github.com/seamusabshere/upsert - the first argument is a hash of attributes that is used to find or create a record – Poff

It is helpful to note that initialize calls the create() instead of new() that might be called in case of first_or_create – Bradfield

Can some one share a gist example of this usage? I'm not familiar with its placement and calling. – Chaim

I reverted the removal of Rails 3 code, since lots of people are not using Rails 4 yet. – Constant

Just to add: find_or_create_by_*() functions have been deprecated in Rails 4, however find_or_create_by() is NOT. It is okay to use find_or_create_by(). Check the commit where this was added github.com/rails/rails/commit/… and the official Rails 4.2 documentation for the usage: guides.rubyonrails.org/v4.2.0/… – Bridgeport

Does the find_or_create_by is really deprecated in Rails 4? I just found a resource apidock.com/rails/v4.2.1/ActiveRecord/Relation/… under version of Rails 4.2.1 – Chemoreceptor

Answer 2

In Rails 4 you could do:

GroupMember.find_or_create_by(member_id: 4, group_id: 7)

And use where is different:

GroupMember.where(member_id: 4, group_id: 7).first_or_create

This will call create on GroupMember.where(member_id: 4, group_id: 7):

GroupMember.where(member_id: 4, group_id: 7).create

On the contrary, the find_or_create_by(member_id: 4, group_id: 7) will call create on GroupMember:

GroupMember.create(member_id: 4, group_id: 7)

Please see this relevant commit on rails/rails.

Dry

Answer 3

For anyone else who stumbles across this thread but needs to find or create an object with attributes that might change depending on the circumstances, add the following method to your model:

# Return the first object which matches the attributes hash
# - or -
# Create new object with the given attributes
#
def self.find_or_create(attributes)
  Model.where(attributes).first || Model.create(attributes)
end

Optimization tip: regardless of which solution you choose, consider adding indexes for the attributes you are querying most frequently.

Absa

One thing to note is that this won't handle attributes that cannot be mass-assigned, while find_or_create_by will. – Racoon

Marco, try Model.where(attributes).instance_eval{|q| q.first || q.create}. – Presbyterian

find_or_create also has the benefit of using a transaction, where where….first || create introduces a race condition – Jeanelle

I would say def self.find_or_create(attributes) self.where(attributes).first || self.create(attributes) end so that you don't need to repeat Model – Nyssa

@AugustinRiedinger You don't need self inside the method body either (since you're looking to remove words!). – Dwanadwane

@mm - Using a transaction here does not prevent a race condition, unless you have configured a higher-than-normal (and less performant) isolation level on your RDBMS. Note that api.rubyonrails.org/classes/ActiveRecord/… specifically mentions that find_or_create_by is vulnerable to a race condition. – Voorhees

Answer 4

By passing a block to find_or_create, you can pass additional parameters that will be added to the object if it is created new. This is useful if you are validating the presence of a field that you aren't searching by.

Assuming:

class GroupMember < ActiveRecord::Base
    validates_presence_of :name
end

then

GroupMember.where(:member_id => 4, :group_id => 7).first_or_create { |gm| gm.name = "John Doe" }

will create a new GroupMember with the name "John Doe" if it doesn't find one with member_id 4 and group_id 7

Ballad

Answer 5

You can do:

User.find_or_create_by(first_name: 'Penélope', last_name: 'Lopez')
User.where(first_name: 'Penélope', last_name: 'Lopez').first_or_create

Or to just initialize:

User.find_or_initialize_by(first_name: 'Penélope', last_name: 'Lopez')
User.where(first_name: 'Penélope', last_name: 'Lopez').first_or_initialize

Raouf

api.rubyonrails.org/classes/ActiveRecord/… – Subcelestial