While One-to-Many and Many-to-One relationships work in Canvas PowerApps, also in Model-Driven Apps and even in Excel add in, Many-to-Many relationship type does not work in canvas PowerApps.
I have the scenario of SupplierEntity, and ProductEntity that I have created a Many-to-Many Supplier <->Product to cover senarios like any Supplier can offer any product - which is my business scenario.
I remember created the relationship in ProductEntity and I can see it the same in SupplierEntity which makes sense.
I use Canvas Apps because Model Driven Apps are not priced fairly for users! so I tried access products for suppliers to create the connection, and trust me there was no way to do it.
Although in the relationship settings I can see RelationshipName Supplier_Product and Relationship Entity Name supplier_product, physically, there have been no entities created in the list of entities that are making the relationship.
Excel add in couldn't make access to any column related to the relationship.
I tried Model Driven Apps and was perfectly there: In Supplier Entity I could add from existing Products a ProductSupplier relationship, while in Product, I also could add a ProductSupplier equally.
There is nothing mentioned in documentation that Canvas Apps will not deal with CDS Many-to-Many relationships.
So what is the way to?
I remember that I have posted an idea 2 years ago on that PowerApps (Canvas) should have the best of Access Web Apps including Related-Items-Control.
This I think has been rejected - because I think it is too complicated to develop such a control to deal with the many data sources PowerApps deal with.
Besides, there is a way to make a simple related items control for Supplier/Contact entities for instance (One to Many) via 2 galleries or datatables or combination.
But here we are: cannot use many to many relationship in Canvas Apps!!
I now think that PowerApps canvas should have 2 versions: one that is existing, and the other one is to support CDS. Or at least there must be new controls added to support CDS exclusively.
Dont tell me then that the answer is Model Driven Apps. Right it is made for CDS only, but it remains a choice, we are never told that the best of CDS is only availbe if we pay USD 40.00 per month per user or per developer alike to use only in model driven apps. this is not a good marketing strategy if it is true.
Appreciate your feedback dear PowerApps team