Introducing Relationship Custom Fields

As we learned in Reviewing Database Concepts, we can define a relationship between two objects through the use of common fields. On the platform, we can define relationships between objects by creating a relationship custom field that associates one object with another. A relationship field is a custom field on an object record that contains a link to another record. When we place a relationship custom field on an object, we're effectively creating a many-to-one relationship between the object on which the relationship field is placed and the other object.

There are different types of relationship fields, each with different implications. The simplest and most flexible type is a lookup relationship field, which creates a simple relationship between two objects. For example, if we place a lookup relationship field on a Job Application object that references position records, many job application records can be related to a single position record. This will be reflected both with a new Position field on the job application record and with a new Job Applications related list on the position record. You can also put multiple lookup relationship fields on a single object, which means that our Job Application object can also point to a Candidate object.

A second type of relationship field, master-detail relationship, is a bit more complex, but more powerful. Master-detail relationships create a special parent-child relationship between objects: the object on which you create the master-detail relationship field is the child or “detail,” and the object referenced in the field is the parent or “master.” In a master-detail relationship, the ownership and sharing of detail records are determined by the master record, and when you delete the master record, all of its detail records are automatically deleted along with it. Master-detail relationship fields are always required on detail records, and once you set a master-detail relationship field's value, you can't change it.

When do you use a master-detail relationship? If you have an object that derives its significance from another object. For example, say you have a Review custom object that contains an interviewer's feedback on a job application. If you delete a job application record, you will probably want all of its review records deleted as well, being that reviews of something that no longer exists aren't very useful. In this case, you want to create a master-detail relationship on the Review custom object with the Job Application object as the master object.

That's the sort of thing that we're going to do in this chapter. First, let's start with the really quick and easy example of putting a Hiring Manager field on our Position object—we'll create a many-to-one relationship between the Position object and the standard User object that comes with every organization, reflecting the fact that a hiring manager can be responsible for several positions at a time. Then we'll build out a few more objects and implement a more complex relationship involving positions, job applications, candidates, and reviews.