Module: ActiveRecord::FinderMethods

Included in:
Relation
Defined in:
activerecord/lib/active_record/relation/finder_methods.rb

Constant Summary collapse

ONE_AS_ONE = 
'1 AS one'

Instance Method Summary collapse

Instance Method Details

#exists?(conditions = :none) ⇒ Boolean

Returns truthy if a record exists in the table that matches the id or conditions given, or falsy otherwise. The argument can take six forms:

  • Integer - Finds the record with this primary key.

  • String - Finds the record with a primary key corresponding to this string (such as '5').

  • Array - Finds the record that matches these find-style conditions (such as ['color = ?', 'red']).

  • Hash - Finds the record that matches these find-style conditions (such as {color: 'red'}).

  • false - Returns always false.

  • No args - Returns false if the table is empty, true otherwise.

For more information about specifying conditions as a Hash or Array, see the Conditions section in the introduction to ActiveRecord::Base.

Note: You can’t pass in a condition as a string (like name = 'Jamie'), since it would be sanitized and then queried against the primary key column, like id = 'name = \'Jamie\''.

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

Returns:

  • (Boolean) 


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# File 'activerecord/lib/active_record/relation/finder_methods.rb', line 200

def exists?(conditions = :none)
  conditions = conditions.id if Base === conditions
  return false if !conditions

  relation = construct_relation_for_association_find(construct_join_dependency)
  return false if ActiveRecord::NullRelation === relation

  relation = relation.except(:select, :order).select(ONE_AS_ONE).limit(1)

  case conditions
  when Array, Hash
    relation = relation.where(conditions)
  else
    relation = relation.where(table[primary_key].eq(conditions)) if conditions != :none
  end

  connection.select_value(relation.arel, "#{name} Exists", relation.bind_values)
end

#find(*args) ⇒ Object

Find by id - This can either be a specific id (1), a list of ids (1, 5, 6), or an array of ids ([5, 6, 10]). If no record can be found for all of the listed ids, then RecordNotFound will be raised. If the primary key is an integer, find by id coerces its arguments using to_i.

Person.find(1)       # returns the object for ID = 1
Person.find("1")     # returns the object for ID = 1
Person.find(1, 2, 6) # returns an array for objects with IDs in (1, 2, 6)
Person.find([7, 17]) # returns an array for objects with IDs in (7, 17)
Person.find([1])     # returns an array for the object with ID = 1
Person.where("administrator = 1").order("created_on DESC").find(1)

ActiveRecord::RecordNotFound will be raised if one or more ids are not found.

NOTE: The returned records may not be in the same order as the ids you provide since database rows are unordered. You’d need to provide an explicit order option if you want the results are sorted.

Find with lock

Example for find with a lock: Imagine two concurrent transactions: each will read person.visits == 2, add 1 to it, and save, resulting in two saves of person.visits = 3. By locking the row, the second transaction has to wait until the first is finished; we get the expected person.visits == 4.

Person.transaction do
  person = Person.lock(true).find(1)
  person.visits += 1
  person.save!
end

Variations of find

Person.where(name: 'Spartacus', rating: 4)
# returns a chainable list (which can be empty).

Person.find_by(name: 'Spartacus', rating: 4)
# returns the first item or nil.

Person.where(name: 'Spartacus', rating: 4).first_or_initialize
# returns the first item or returns a new instance (requires you call .save to persist against the database).

Person.where(name: 'Spartacus', rating: 4).first_or_create
# returns the first item or creates it and returns it, available since Rails 3.2.1.

Alternatives for find

Person.where(name: 'Spartacus', rating: 4).exists?(conditions = :none)
# returns a boolean indicating if any record with the given conditions exist.

Person.where(name: 'Spartacus', rating: 4).select("field1, field2, field3")
# returns a chainable list of instances with only the mentioned fields.

Person.where(name: 'Spartacus', rating: 4).ids
# returns an Array of ids, available since Rails 3.2.1.

Person.where(name: 'Spartacus', rating: 4).pluck(:field1, :field2)
# returns an Array of the required fields, available since Rails 3.1.



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# File 'activerecord/lib/active_record/relation/finder_methods.rb', line 63

def find(*args)
  if block_given?
    to_a.find { |*block_args| yield(*block_args) }
  else
    find_with_ids(*args)
  end
end

#find_by(*args) ⇒ Object

Finds the first record matching the specified conditions. There is no implied ordering so if order matters, you should specify it yourself.

If no record is found, returns nil.

Post.find_by name: 'Spartacus', rating: 4
Post.find_by "published_at < ?", 2.weeks.ago



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# File 'activerecord/lib/active_record/relation/finder_methods.rb', line 79

def find_by(*args)
  where(*args).take
end

#find_by!(*args) ⇒ Object

Like find_by, except that if no record is found, raises an ActiveRecord::RecordNotFound error.



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# File 'activerecord/lib/active_record/relation/finder_methods.rb', line 85

def find_by!(*args)
  where(*args).take!
end

#first(limit = nil) ⇒ Object

Find the first record (or first N records if a parameter is supplied). If no order is defined it will order by primary key.

Person.first # returns the first object fetched by SELECT * FROM people
Person.where(["user_name = ?", user_name]).first
Person.where(["user_name = :u", { u: user_name }]).first
Person.order("created_on DESC").offset(5).first
Person.first(3) # returns the first three objects fetched by SELECT * FROM people LIMIT 3

Rails 3

Person.first # SELECT "people".* FROM "people" LIMIT 1

NOTE: Rails 3 may not order this query by the primary key and the order will depend on the database implementation. In order to ensure that behavior, use User.order(:id).first instead.

Rails 4

Person.first # SELECT "people".* FROM "people" ORDER BY "people"."id" ASC LIMIT 1



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# File 'activerecord/lib/active_record/relation/finder_methods.rb', line 127

def first(limit = nil)
  if limit
    find_first_with_limit(limit)
  else
    find_first
  end
end

#first! ⇒ Object

Same as first but raises ActiveRecord::RecordNotFound if no record is found. Note that first! accepts no arguments.



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# File 'activerecord/lib/active_record/relation/finder_methods.rb', line 137

def first!
  first or raise RecordNotFound
end

#last(limit = nil) ⇒ Object

Find the last record (or last N records if a parameter is supplied). If no order is defined it will order by primary key.

Person.last # returns the last object fetched by SELECT * FROM people
Person.where(["user_name = ?", user_name]).last
Person.order("created_on DESC").offset(5).last
Person.last(3) # returns the last three objects fetched by SELECT * FROM people.

Take note that in that last case, the results are sorted in ascending order:

[#<Person id:2>, #<Person id:3>, #<Person id:4>]

and not:

[#<Person id:4>, #<Person id:3>, #<Person id:2>]



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# File 'activerecord/lib/active_record/relation/finder_methods.rb', line 156

def last(limit = nil)
  if limit
    if order_values.empty? && primary_key
      order(arel_table[primary_key].desc).limit(limit).reverse
    else
      to_a.last(limit)
    end
  else
    find_last
  end
end

#last! ⇒ Object

Same as last but raises ActiveRecord::RecordNotFound if no record is found. Note that last! accepts no arguments.



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# File 'activerecord/lib/active_record/relation/finder_methods.rb', line 170

def last!
  last or raise RecordNotFound
end

#raise_record_not_found_exception!(ids, result_size, expected_size) ⇒ Object

This method is called whenever no records are found with either a single id or multiple ids and raises a ActiveRecord::RecordNotFound exception.

The error message is different depending on whether a single id or multiple ids are provided. If multiple ids are provided, then the number of results obtained should be provided in the result_size argument and the expected number of results should be provided in the expected_size argument.

Raises:



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# File 'activerecord/lib/active_record/relation/finder_methods.rb', line 227

def raise_record_not_found_exception!(ids, result_size, expected_size) #:nodoc:
  conditions = arel.where_sql
  conditions = " [#{conditions}]" if conditions

  if Array(ids).size == 1
    error = "Couldn't find #{@klass.name} with #{primary_key}=#{ids}#{conditions}"
  else
    error = "Couldn't find all #{@klass.name.pluralize} with IDs "
    error << "(#{ids.join(", ")})#{conditions} (found #{result_size} results, but was looking for #{expected_size})"
  end

  raise RecordNotFound, error
end

#take(limit = nil) ⇒ Object

Gives a record (or N records if a parameter is supplied) without any implied order. The order will depend on the database implementation. If an order is supplied it will be respected.

Person.take # returns an object fetched by SELECT * FROM people LIMIT 1
Person.take(5) # returns 5 objects fetched by SELECT * FROM people LIMIT 5
Person.where(["name LIKE '%?'", name]).take



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# File 'activerecord/lib/active_record/relation/finder_methods.rb', line 96

def take(limit = nil)
  limit ? limit(limit).to_a : find_take
end

#take! ⇒ Object

Same as take but raises ActiveRecord::RecordNotFound if no record is found. Note that take! accepts no arguments.



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# File 'activerecord/lib/active_record/relation/finder_methods.rb', line 102

def take!
  take or raise RecordNotFound
end