Creating a Many-to-Many Relationship

Our Recruiting app now has quite a few many-to-one relationships, but what if we needed to create a many-to-many relationship? For example, what if we have an object that stored information about various employment websites, and we wanted to track which open positions we posted to those sites? This would require a many-to-many relationship because:

• One position could be posted on many employment websites.
• One employment website could list many positions.

Here's where we get a little creative. Instead of creating a relationship field on the Position object that directly links to the Employment Website object, we can link them using a junction object. A junction object is a custom object with two master-detail relationships, and is the key to making a many-to-many relationship.

For our app, we're going to create a junction object called Job Posting. A job posting fits into the space between positions and employment websites—one position can be posted many times, and one employment website can have many job postings, but a job posting always represents a posting about a single position on a single employment website. In essence, the Job Posting object has a many-to-one relationship with both the Position and the Employment Website objects, and through those many-to-one relationships, we'll have a many-to-many relationship between the Position and Employment Website objects.

Let's look at a typical scenario at Universal Containers. There are open positions for a Project Manager and a Sr. Developer. The Project Manager position is only posted on Monster.com, but the Sr. Developer position is more difficult to fill, so it's posted on both Monster.com and Dice. Every time a position is posted, a job posting record tracks the post. As you can see in the following diagram, one position can be posted many times, and both positions can be posted to the same employment website.

Using a Job Posting Object to Create a Many-to-Many Relationship Between Positions and Employment Websites

In relational database terms, each job posting record is a row in the Job Posting table consisting of a foreign key to a position record and a foreign key to an employment website record. The following entity relationship diagram shows this relationship.

Entity Relationship Diagram for the Position, Job Posting, and Employment Website Objects