find

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 one or more records cannot be found for the requested ids, then ActiveRecord::RecordNotFound will be raised. If the primary key is an integer, find by id coerces its arguments by using to_i.

Person.find(1)          # returns the object for ID = 1
Person.find("1")        # returns the object for ID = 1
Person.find("31-sarah") # returns the object for ID = 31
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)

NOTE: The returned records are in the same order as the ids you provide. If you want the results to be sorted by database, you can use ActiveRecord::QueryMethods#where method and provide an explicit ActiveRecord::QueryMethods#order option. But ActiveRecord::QueryMethods#where method doesn’t raise ActiveRecord::RecordNotFound.

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

Person.find_or_create_by(name: 'Spartacus', rating: 4)
# returns the first item or creates it and returns it.

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.

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