Symfony 4. ManyToMany association with attribute. Custom form to insert attribute value, how do I?

Question

A Symfony 4 project. I am not able to see ho to code the following spec.

Say I have two entities Alpha and Beta. When I create an Alpha object alpha, I wish to associate (ManyToMany) to it one or more Beta objects. I know how to render new and edit Forms to do so.

I wish to enrich the AlphaBeta join table with an attribute, say the Cost to associate a Beta to an Alpha. The issue is that I am not able to enrich the forms above as to insert or edit a Cost value, when I create a Alpha object and associate to it a Beta object.

Is there a standard way to code a situation of this kind?

I read that the way to go is to have two OneToMany associations Alpha->AlphaBeta, and Beta->AlphaBeta, but even doing so I am not able to define/render a Form for Alpha as to create a new Alpha object, to associate to it a Beta object (or more) and to assign to such association a Cost value.

Your advise is very welcome. Thanks

Answer 1

Generally, if the relation/association itself should have an attribute, then the many-to-many mapping in doctrine isn't sufficient anymore, since associations can't store additional data. So you correctly note, that you need an extra AlphaBeta entity/class that holds the data. (As already posted by Ali Kazemi, there is a tutorial for this)

But you wonder about how the cost field can be added to your form...

Since the cost is part of the AlphaBeta entity/class in your case, the form field should be in a form type AlphaBetaType that - probably depending on the options provided - should render an AlphaType and/or BetaType sub form, and a cost form field. Custom form themeing can display it in a way, that it doesn't appear as if it was a subform, if that is a concern. (However, it should be noted that custom form theming can be annoying at times...)

In general, the form structure/hierarchy usually very much resembles the entity structure/hierarchy. And only sporadically hiding data or mapping/transforming it to be displayed or handled differently.

Alternatively you can add an unmapped form field and later store that in your AlphaBeta, but that on average isn't simpler, since it involves "manual" handling.