What's the difference between find, where and find_by_id?

The difference is what they return when a record is found, or when it's not found. Consider the following examples:

>> User.create name: 'THE USER' # creates a user with id = 1
>> User.find(1) # returns the user
>> User.find_by_id(1) # returns the user
>> User.where(id: 1).first # returns the user

As you can see, an existing user can be fetched using any of the 3 methods. The big difference with using where is you can chain commands (of course, without calling first first.)

Let's have a look at when you try to find a record that isn't existing

>> User.find(2) # raises an exception
>> User.find_by_id(2) # nil
>> User.where(id: 2).first # nil

So here, it's obvious that when you use find to search for a record that isn't existing, you get an exception. That exception is ActiveRecord::RecordNotFound which renders a 404 on production environment.

Hope this helps!

UPDATE

Rails 4 uses the following syntax for find_by

>> User.find_by(id: 1)  # returns nil if there's no user with an id of 1
>> User.find_by!(id: 1) # raises ActiveRecord::RecordNotFound when no record is found

find => This return single record if the given primary_key(id) exists in the system otherwise in will give an error.

Model.find(required_id_value)

find_by => This will return single record depends on the given attribute, and if the value of the attribute is not exist in the DB it will return nil.

Model.find_by_name("your name")

name here is the attribute and it must be exist in your Modal.

where => This will return an active record relation with zero or more records you need to use first to return only one record or nil in case zero records return.

Model.where(id: id_value).first