Rails find_or_create_by more than one attribute?

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.

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)

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.