Custom Module & Relationshipbuilder for VTiger is an extension that allows crm administrators to create custom modules within few clicks. All custom modules are created following strict VTiger standards. In addition, the relationship builder allows crm admin to link together existing modules as well as new custom modules. It supports 1:1, 1:M and M:M relationships.
How to Custom Add M:M Relationship
Go to Menu > Settings > CRM Settings > Extension Pack > Custom Module Builder.
Click on “M:M Relationship” button to add a relation.
Select one of the modules you want to interlink in the many-to-many relationship.
Select the other module you want to link to the first module in a many-to-many relationship.
You can see the Tab in the 2nd module that is populated from the 1st module.