实体框架：一对多关系

Question

I have a design problem with regards to Entity Framework model relationship

I have this model in the edmx

Business Rule:

A Participant can have multiple Roles so I create a relationship table ParticipantRoles that has 1-to-Many relationship on the Participant and the Role table

The Problem : In order to get the Participant's Role value, I have to drill down through Participant->ParticipantRole->Role (see JSON output below)

The Question:

In EF, how to design the table relationship to bypass the ParticipantsRole table. I want to access the Role in something like this Particant.Role and not Participant.ParticipantsRole.Role

Answer 1

You say A Participant can have multiple Role s . And of course, a Role can have multiple Participant s. So basically this is a many-to-many association.

Entity Framework will only map pure many-to-many associations (without connecting class) when the junction table only has two foreign keys. In your case, if the table ParticipantsRole only would have had a primary key consisting of ParticipantId and RoleId at the time of generating the model the class ParticipantsRole would not have been created. You would have had Participant.Roles and Role.Participants as navigation properties.

However, the model has been generated with ParticipantsRole and you want to get rid of it. (Or not, I'll get back to that).

This is what you can do:

• Remove ParticipantRoles from the class diagram.
• Modify the database table ParticipantRoles so it only has the two FK columns, that both form the primary key.
• Update the model from the database and select ParticipantsRole in the Add tab.

This should give you a model with a pure many-to-many association.

However, think twice before you do this. M2m associations have a way of evolving into 1-m-1association (as you've got now). The reason is that sooner or later the need is felt to record data about the association, so the junction table must have more fields and stops being a pure junction table. In your case I can imagine that one day participant's roles must have a fixed order, or one marked as default. It can be a major overhaul to change a m2m association into 1-m-1 in a production environment. - Something to consider...