Rails idiom to avoid duplicates in has_many :through
Solution 1
As long as the appended role is an ActiveRecord object, what you are doing:
user.roles << role
Should de-duplicate automatically for :has_many
associations.
For has_many :through
, try:
class User
has_many :roles, :through => :user_roles do
def <<(new_item)
super( Array(new_item) - proxy_association.owner.roles )
end
end
end
if super doesn't work, you may need to set up an alias_method_chain.
Solution 2
Use Array's |=
Join Method.
You can use Array's |=
join method to add an element to the Array, unless it is already present. Just make sure you wrap the element in an Array.
role #=> #<Role id: 1, name: "1">
user.roles #=> []
user.roles |= [role] #=> [#<Role id: 1, name: "1">]
user.roles |= [role] #=> [#<Role id: 1, name: "1">]
Can also be used for adding multiple elements that may or may not already be present:
role1 #=> #<Role id: 1, name: "1">
role2 #=> #<Role id: 2, name: "2">
user.roles #=> [#<Role id: 1, name: "1">]
user.roles |= [role1, role2] #=> [#<Role id: 1, name: "1">, #<Role id: 2, name: "2">]
user.roles |= [role1, role2] #=> [#<Role id: 1, name: "1">, #<Role id: 2, name: "2">]
Found this technique on this StackOverflow answer.
Solution 3
You can use a combination of validates_uniqueness_of and overriding << in the main model, though this will also catch any other validation errors in the join model.
validates_uniqueness_of :user_id, :scope => [:role_id]
class User
has_many :roles, :through => :user_roles do
def <<(*items)
super(items) rescue ActiveRecord::RecordInvalid
end
end
end
Solution 4
i think the proper validation rule is in your users_roles join model:
validates_uniqueness_of :user_id, :scope => [:role_id]
KingPong
Polyglot programmer and automation zealot. Currently automating massively scalable architecture for The Weather Company.
Updated on June 06, 2022Comments
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KingPong almost 2 years
I have a standard many-to-many relationship between users and roles in my Rails app:
class User < ActiveRecord::Base has_many :user_roles has_many :roles, :through => :user_roles end
I want to make sure that a user can only be assigned any role once. Any attempt to insert a duplicate should ignore the request, not throw an error or cause validation failure. What I really want to represent is a "set", where inserting an element that already exists in the set has no effect. {1,2,3} U {1} = {1,2,3}, not {1,1,2,3}.
I realize that I can do it like this:
user.roles << role unless user.roles.include?(role)
or by creating a wrapper method (e.g.
add_to_roles(role)
), but I was hoping for some idiomatic way to make it automatic via the association, so that I can write:user.roles << role # automatically checks roles.include?
and it just does the work for me. This way, I don't have to remember to check for dups or to use the custom method. Is there something in the framework I'm missing? I first thought the :uniq option to has_many would do it, but it's basically just "select distinct."
Is there a way to do this declaratively? If not, maybe by using an association extension?
Here's an example of how the default behavior fails:
>> u = User.create User Create (0.6ms) INSERT INTO "users" ("name") VALUES(NULL) => #<User id: 3, name: nil> >> u.roles << Role.first Role Load (0.5ms) SELECT * FROM "roles" LIMIT 1 UserRole Create (0.5ms) INSERT INTO "user_roles" ("role_id", "user_id") VALUES(1, 3) Role Load (0.4ms) SELECT "roles".* FROM "roles" INNER JOIN "user_roles" ON "roles".id = "user_roles".role_id WHERE (("user_roles".user_id = 3)) => [#<Role id: 1, name: "1">] >> u.roles << Role.first Role Load (0.4ms) SELECT * FROM "roles" LIMIT 1 UserRole Create (0.5ms) INSERT INTO "user_roles" ("role_id", "user_id") VALUES(1, 3) => [#<Role id: 1, name: "1">, #<Role id: 1, name: "1">]
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KingPong over 14 yearsThanks. That doesn't actually do what I want though (which is a set-like behavior), and I've clarified what that is in the original post. Sorry 'bout that.
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KingPong over 14 yearsIt doesn't work like that. I'll update the post to include the test.
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austinfromboston over 14 yearsI think this is the best answer for your problem. If you are careful in creating your interface, a user would have to hack it to add the wrong role anyway, in which case a validation exception is a totally suitable response.
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KingPong over 14 yearsHeh, are you crazy? Users don't add their own roles :-) The typical use case is that a user becomes a member of a role as a side effect of something else. For example, buying a particular product. Other products may also provide the same role, so there is a chance for duplication there. I'd rather do the duplication checking in one place than in whatever random places need to ensure a user has a role. In this sense, giving a user a role he already has is NOT an error condition.
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KingPong over 14 yearsThanks, I'll try the association extension.
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KingPong over 14 yearsThat worked perfectly. Thanks! The part I was missing when I tried something like this myself was the proxy_owner bit.
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KingPong over 14 yearsFor posterity, the above method can be shortened and genericized to: def <<(*items) super(items - proxy_target) end
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austinfromboston over 14 yearsThat was my first version :). I didn't have actual models set up and was concerned that super(nil) might cause errors. updating the answer with your version and leaving the sub-par code here ( for posterity ): def <<(*items); new_items = items - proxy_owner.roles; super( new_items ) unless new_items.empty?; end
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Turadg over 11 yearsFor Rails 3.1,
s/proxy_owner/proxy_association.owner/
related Q -
Turadg over 11 yearsWhy make the argument
*items
when<<
takes only a single object? ruby-doc.org/core-1.9.3/Array.html#method-i-3C-3C -
austinfromboston over 11 yearsIt's an easy way to transparently convert the added item to an array. I'll update it with the new idioms.
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Turadg over 11 yearsIt's odd to me that << dedupes for
has_many
but nothas_many :through
. However a Rails issue to fix this (github.com/rails/rails/issues/8573) was rejected with "This is your domain logic so it your responsibility to check it." -
Ashitaka over 10 yearsCouldn't you change that exception to
ActiveRecord::RecordNotUnique
? I like this answer. Be aware of race conditions though. -
Ruby Racer about 8 yearsNice answer. I used it without
validates_uniqueness_of
, having declared unique index in database and works charmingly. -
lulalala over 7 years@Turadg Now
<<
can take more than one argument guides.rubyonrails.org/… -
DonMB almost 3 yearsCorrect and most clean answer in my opinion