代码优先实体框架-多对多关系

Question

I have the following

class Human
  int id
  string gender
  List<PhysicalAttribute> attributes

class PhysicalAttributes
  int id
  string description
  string type
  List<Human> humans

When I add the first human with attributes, the tables are created and the data is populated properly.

The problem is when I add the next humans with similar attributes. Let's say I have attributes

type:"body"
description:"slim"

for both the first and second human. When I create new and add the second human, another entry with the same type and description is added to the PhysicalAttributes table.

Is there some way of doing this so that the existing entry will be used?

Do I have to do a lookup on the PhysicalAttributes table first to see that the entry has been created?

Answer 1

Is there some way of doing this so that the existing entry will be used?

Yes. Make (type,description) the Entity Key for PhysicalAttributes. You seem to have introduced a meaningless ID key property that allows multiple PhysicalAttributes to have the same type and description.

It is also possible to not fix the model, and fetch the existing PhysicalAttributes from the database to discover if any of the ones you need already exist. But that's just a tedious workaround for having the wrong entity key structure.

If you are loading from JSON it's going to be inconventient to attach existing entities, and instead you can do the following.

Override SaveChanges and fetch all the PhyscialAttribute values from the database. Then for the PhysicalAttribute entities that already exist in the database, Detach them from the DbContext, and EF won't attempt to insert them.

After you call PhysicalAttributes.Load(), you will have two entries in the ChangeTracker for every PhysicalAttribute on a new Entity that already exists in the database.

EG

    public override int SaveChanges()
    {

        PhysicalAttributes.Load();

        var entryGroup = from entry in ChangeTracker.Entries<PhysicalAttribute>()
                      group entry by new { entry.Entity.Description, entry.Entity.Type } into byKey
                      where byKey.Any( e => e.State == EntityState.Unchanged)
                      select byKey;

        foreach (var eg in entryGroup)
        {
            foreach (var e in eg )
            {
                if (e.State == EntityState.Added)
                {
                    e.State = EntityState.Detached;
                }
            }
        }

        return base.SaveChanges();
    }