method

exists?

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What's this?

exists?(id_or_conditions) public

Checks whether a record exists in the database that matches conditions given. These conditions can either be a single integer representing a primary key id to be found, or a condition to be matched like using ActiveRecord#find.

The id_or_conditions parameter can be an Integer or a String if you want to search the primary key column of the table for a matching id, or if you’re looking to match against a condition you can use an Array or a Hash.

Possible gotcha: You can’t pass in a condition as a string e.g. "name = ‘Jamie’", this would be sanitized and then queried against the primary key column as "id = ‘name = \’Jamie"

Examples

  Person.exists?(5)
  Person.exists?('5')
  Person.exists?(:name => "David")
  Person.exists?(['name LIKE ?', "%#{query}%"])

Show source

# File activerecord/lib/active_record/base.rb, line 654
      def exists?(id_or_conditions)
        connection.select_all(
          construct_finder_sql(
            :select     => "#{quoted_table_name}.#{primary_key}",
            :conditions => expand_id_conditions(id_or_conditions),
            :limit      => 1
          ),
          "#{name} Exists"
        ).size > 0
      end

February 3, 2009

3 thanks

Possible gotcha

Please note that exists? doesn’t hold all the conventions of find, i.e. you can’t do:

Person.exists?(:conditions => ['name LIKE ?', "%#{query}%"]) # DOESN'T WORK!

January 30, 2009 - (<= v2.1.0)

3 thanks

Hash conditions require explicit key and value

When condition passed as hash, the behavior is different from a finder method. Finder methods, such as:

find(:all, :user=>user)

will apply the user_id = user.id convention, provided user is an association (e.g. belongs_to :user). The exists? method will not do the same. You must specify the foreign key name and value explicitly, i.e:

exists?(:user_id=>user.id)

September 9, 2010

2 thanks

takes ActiveRecord object as an arg as well

One undocumented feature, you can do this:

person = Person.first
Person.exists?(person)
# => returns true

This came in handy for me when I needed to see if something belonged to a particular scope.

scope = "created_rails"
person = Person.find_by_name "dhh"
Person.send(scope).exists?(person)
# => returns true

Obviously this relies on you having a named_scope in your Person model called “created_rails”.

April 22, 2010

2 thanks

Dynamic exists? methods

There are no dynamic exists? methods analogous to dynamic finders, which means that while you can do this:

Person.find_by_name('David')

you can’t do this:

Person.exists_by_name('David') # DOES NOT WORK

nor this:

Person.exists_by_name?('David') # DOES NOT WORK

However, you can simulate this with dynamic scope:

Person.scoped_by_name('David').exists?

You’ll have to admit that this is so much better than the plain old method:

Person.exists?(:name => "David")