A junction object is a custom object with two master-detail relationships, and it is the key to making a many-to-many relationship. This sentence you will find in many salesforce official knowledge articles but it doesn’t necessarily have to be just master detail relationship. You can create a junction object using lookup relationship as well.

Though a MD relationship is preferred because :

  • In lookup, a child record can exist independently thereby defeating the purpose of junction object.
  • No Cascade Delete
  • Independent record ownership and owd.

Junction objects play a very vital role in some scenarios where we need many-to-many relationships to solve a business problem. Consider an example where we have students and sports are two objects. A sport can be played by many students and similarly a student can play multiple sports.

A many-to-many relationship is created by defining a third table which is called a junction object. In Salesforce, we can define this third table which can have the following combinations defined with the other two tables:

  1. Two lookup relationships
  2. Two Master-Detail relationships
  3. One lookup and one master detail relationship

Let’s take an example and see how to create junction object:

  1. Create an object named X.
  2. Created an object named Y.
  3. Create a junction object named Z, which will have two lookup relationships or two MD relationships with X and Y, respectively. It can also have one master-detail relationship and one lookup relationship as well.

Important Things to note :

  • A junction object cannot become master object in multilevel MD relationships.
  • Outbound messages not allowed.

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