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First you'll need to add a Summary field (To learn how, please see our Create a Summary Field help topic.) on the Master table to count the number of related Detail records which are assigned to the current user:
1. Open the Master table, and click Settings.
2. Choose Table-to-table relationships and select the relationship with the Details table you're using to check the permissions.
3. Click Add Summary Field. "Create a new field in the Projects table displaying" is already set correctly, so you can leave that.
4. Change the Matching Criteria so it reads: "Only summarize records where the following is true in <Detail>": <Some User-type field> "is the current user"
5. Click Create, re-name the field if you want, then click OK.
Next you'll need to create a custom view rule on the Master table:
1. Return to the Home page of the app, and click Users.
2. Choose Manage Roles, and click the name of the role you're working on.
3. On the row for the Master table, select Custom Rule under the View column. A pop-up window appears.
4. Change the rule so it reads:
5. Click Save to finish.
How does the above work?
The Summary field created in the Master table is greater than zero only for those master records which have at least one Details table record whose specified User-type field value is the user who is signed in, i.e. "me". Hence the user who is signed in can only view those records in the Master table.
Possible application examples:
Allowing a user to view only those project records which have one or more tasks with the [Assigned To] field value set to the user. Or, allowing a user to view only those contact master records which have one or more opportunities assigned to the user.
For more on custom user permissions in general, please see our Configure Permissions for a Role help topic.