Get ID of Rails Model before saving…?

Question

How do you get the id of a rails model before it is saved?

For example, if I create a new model instance, how can I get its ID before it is saved?

I know t

Answer 1

Most of the time when I needed an id can be grouped into a short list. When creating nested associations or connectin of the associations through. Let's assume we have: :user that have :pets through :user_pets association, where we will save their type.

If we have a properly configured "has_many: through Association" we can just User.pets.create(name: "Rex") but this is too simplistic, as we want to creat :pet type in :user_pets.

u = User.create(name: "Cesar")
u.id # => 1 # works fine

p = u.pets.create(name: 'Rex') 
# => rails will create UserPets => {id: 1, user_id: 1, pet_id: 1} for us

# But now we have a problem, how do we find :user_pets of our :pet?
# remember we still need to update the :type, the ugly (wrong) way:
up = p.user_pets.first
up.type = 'dog'
up.save! # working but wrong

# Do you see the problems here? We could use an id
P = Pet.new( name: "Destroyer" )
p.id # will not work, as the pet not saved yet to receive an id
up = UserPet.new( user_id: U.id, pet_id: p.id ) 
# => UserPet {id: 2, user_id: 1, pet_id: nil} # as we expected there is no id.

# What solutions do we have? Use nested creation!
# Good
up = UserPet.new(user_id: u.id, type: "dog")
# even better
up = u.user_pets.new(type: "dog") 
# it's just a shortcut for the code above, 
# it will add :user_id for us, so let's just remember it.

# Now lets add another 'new' from our creatd 'user_pet'
p = up.pets.new(name: "Millan")
user.save!
# => UserPet: {id: 3, user_id: 1, pet_id: 2, type: 'dog'} # => Pet: {id: 2, name: "Sam"}
# everything is working! YEY!

# we can even better, than writing in the beginning "User.create", 
# we can write "User.new" and save it with all the nested elements.

You saw how this created all the ids for us? Let's move to something even more complex! Now we have an additional table :shampoo that exactly as :user_pet, belongs to a :pet and a :user We need to create it without knowing the id of the :user and :pet

u = User.new('Mike')
up = u.user_pets.new(type: "cat") 
p = up.pets.new(name: "Rowe")

# But what are we doing now?
# If we do:
s = u.shampoos.new(name: "Dirty Job") 
# => Shampoo: {user_id: 2, pet_id: nil, name: "..."}
# We get "pet_id: nil", not what we want.

# Or if we do:
s = p.shampoos.new(name: "Schneewittchen") 
# => Shampoo: {user_id: nil, pet_id: 3, name: "..."}
# We get "user_id: nil", in both cases we do not get what we want.

# So we need to get the id of not created record, again.
# For this we can just do as in the first example (order is not important)
s = u.shampoos.new(name: "Mission Impossible") 
# => Shampoo: {id: 3, user_id: 2, pet_id: nil, name: "..."}
s.pet = p # this will give the missing id, to the shampoo on save.
# Finish with save of the object:
u.save! #=> Shampoo: {id: 3, user_id: 2, pet_id: 3, name: '...'} # => Pet: ...
# Done!

I tried to cover most common causes when you need element id, and how to overcome this. I hope it will be useful.