Junction Object

In a relational database, a Lookup field in a source object can point to exactly one record in a target object. That capability creates one-to-many and many-to-one relationships. (For example, many Orders point to a single Customer. That's many-to-one. Looking at it the other way around, a Customer has multiple Orders. That's one-to-many.)

But sometimes, you need a many-to-many relationship. (For example, an Order can have multiple tags, and each tag can clearly apply to multiple orders.) You accomplish that goal with a junction object:

Here, the junction object is Orders_Tags. Every record in it points to exactly one Order and one Tag. But several records that point to the same Order can each point to a different Tag. Similarly, several records that point to the same Tag can each point to a different Order. In that way, a many-to-many (N:M) relationship is established.

Create a Junction Object Manually

You can use the platform's Wizard to create a Many to Many relationship. In that case, the Junction Object is created for you, behind the scenes.

You can also create a junction object manually (a technique you'll need to know if you creating relationships programmatically):

1. Create the object that will become the junction. For example: Orders_Tags.
2. Add a Lookup to one of the target objects. For example: Orders.
3. Go to the Relationships tab.
4. Edit the relationship.
5. Select the Junction radio button to make the relationship many-to-many.
6. Specify the object at the other end of the Junction, and the field to use for indexing.
   (Typically, the default ID field is perfect.)

That process creates the second Lookup in the middle object (Orders_Tags), and identifies it as a "junction" object. Because the platform knows it is a junction object, the user sees records from the end-point object (Tags) when doing a look up. (Otherwise, the user would see Orders_Tags.)