If you are new to customizing Dynamics 365 environment and have been creating N:N Relationships between 2 entities and you are not able to add them in the sub-grid. Let’s see below what are you missing –

Scenario

You have created an N:N relationship in your environment between 2 entities but you still can’t see the option to add it to the sub-grid on the form

1. Let’s see the relationship you have created below –
   
   
2. But, you can’t seem to find it on the Form Editor while adding a sub-grid on either of the entities.
   Let’s look at the Opportunity form for example
   Here’s where I want to put my grid for the Reviewers i.e. the N:N Relationship between User and Opportunity
   
   
3. But you are not able to see it on the selection while trying to add it to the grid –
   (The one appearing in selection by default is not the one we are looking for)
   
   

Now, let’s see what needs to be done to make the relationship selectable here.

Display Option

The reason you are not able to see the N:N relationship is because you had not set the Display Option for the Relationship you created –

1. Open the relationship itself.
   
   You’ll notice that the Display option is set to Do not Display in your relationship editor.
   
   
2. Now, let’s set this to Custom Name and name each of the relationship with the purpose of the relationship you had created this for. Using Custom Name is recommended since you might create multiple N:N Relationships between the same 2 entities based on the requirements –
   In this example, I’m giving the Users entity side a name which is ‘Review Users‘. This will help me identify it while adding it on the sub-grid.
   
   
3. The other options available are –
   
   
4. Now, once I save the publish the changes, the same relationship will appear in my list while adding sub-grid.
   
   
   

Hope this was useful!

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Thank you!