Is it possible to negate a scope in Rails 3?

问题:

I have the following scope for my class called Collection:

scope :with_missing_coins, joins(:coins).where("coins.is_missing = ?", true)

I can run Collection.with_missing_coins.count and get a result back -- it works great! Currently, if I want to get collections without missing coins, I add another scope:

scope :without_missing_coins, joins(:coins).where("coins.is_missing = ?", false)

I find myself writing a lot of these "opposite" scopes. Is it possible to get the opposite of a scope without sacrificing readability or resorting to a lambda/method (that takes true or false as a parameter)?

Something like this:

Collection.!with_missing_coins

回答1:

I wouldn't use a single scope for this, but two:

scope :with_missing_coins, joins(:coins).where("coins.is_missing = ?", true)
scope :without_missing_coins, joins(:coins).where("coins.is_missing = ?", false)

That way, when these scopes are used then it's explicit what's happening. With what numbers1311407 suggests, it is not immediately clear what the false argument to with_missing_coins is doing.

We should try to write code as clear as possible and if that means being less of a zealot about DRY once in while then so be it.

回答2:

In Rails 4.2, you can do:

scope :original, -> { ... original scope definition ... }
scope :not_original, -> { where.not(id: original) }

It'll use a subquery.

回答3:

There's no "reversal" of a scope per se, although I don't think resorting to a lambda method is a problem.

scope :missing_coins, lambda {|status| 
  joins(:coins).where("coins.is_missing = ?", status) 
}

# you could still implement your other scopes, but using the first
scope :with_missing_coins,    lambda { missing_coins(true) }
scope :without_missing_coins, lambda { missing_coins(false) }

then:

Collection.with_missing_coins
Collection.without_missing_coins

回答4:

this might just work, did not test it much. uses rails 5 I guess rails 3 has where_values method instead of where_clause.

scope :not, ->(scope_name) do 
              query = self
              send(scope_name).joins_values.each do |join|
                   query = query.joins(join)
              end
              query.where((send(scope_name).
                    where_clause.send(:predicates).reduce(:and).not))
end

usage

Model.not(:scope_name)

回答5:

Update . Now Rails 6 adds convenient and nifty negative enum methods.

# All inactive devices
# Before
Device.where.not(status: :active)
#After 
Device.not_active

Blog post here