Finding records by id

Edit: This article is a bit dated, since Rails 3 prefers the where method to the find method.

Rails has a number of distinct ways to find ActiveRecord objects by id. They are:

Object.find(id)

Object.find_by_id(id)

Object.find_all_by_id(id)

How do you know which one to use? Does it matter?

I found that all 3 methods have their own unique properties, and can be used when you’re looking for specific types of results. The differences can be expressed as follows:

1. Returning an ActiveRecord error or nil – The basic ‘find’ method throws an ActiveRecord::RecordNotFound error if you input an id that does not correspond to object. The other two methods return nil.

2. Whether multiple objects can be returned – The ‘find_by’ syntax only returns a single object. In contrast, the ‘find’ and ‘find_all_by_’ can return multiple objects. If you input multiple id’s into a find_by method, it will simply return the first result.

3. Whether objects are always returned as an array – For the ‘find_all_by’ method, the value is always returned in an array – even if the result is a single object.

In summary:

Method / Result	Error or Nil?	Number of Objects it can Return	Always returns an array?
.find	ActiveRecord error	Multiple	No
.find_by_	nil	Single	No
.find_all_by_	nil	Multiple	Yes, even for a single object.