Locating and Traversing Records

In the Depot application, LineItems have direct relationships to three other models: Cart, Order, and Product. Additionally, models can have indirect relationships mediated by resource objects. The relationship between Orders and Products through LineItems is an example of such a relationship.

All of this is made possible through IDs.

Identifying individual rows

Active Record classes correspond to tables in a database. Instances of a class correspond to the individual rows in a database table. Calling Order.find(1), for instance, returns an instance of an Order class containing the data in the row with the primary key of 1.

If we’re creating a new schema for a Rails application, we’ll probably want to go with the flow and let it add the id primary key column to all our tables. However, if we need to work with an existing schema, Active Record gives us a way of overriding the default name of the primary key for a table.

For example, we may be working with an existing legacy schema that uses the ISBN as the primary key for the books table.

We specify this in our Active Record model using something like the following:

class LegacyBook < ApplicationRecord 
   self.primary_key = "isbn"
end

Normally, Active Record takes care of creating new primary key values for records that we create and add to the database. They’ll be ascending integers, possibly with some gaps in the sequence. However, if we override the primary key column’s name, we take on the responsibility of setting the primary key to a unique value before we save a new row. Perhaps surprisingly, we still set an attribute called id to do this. As far as Active Record is concerned, the primary key attribute is always set using an attribute called id. The primary_key= declaration sets the name of the column to use in the table. In the following code, we use an attribute called id even though the primary key in the database is isbn: