EF - associated entity is trying to add to the database when adding an entity

问题

This question is related to: Saving an explicit entity through Entity Framework

Basically, Jcl's approached worked me. But there's another issue when trying to save the changes, because it tries to insert a duplicate entity which I have set as a foriegn key.

I'm reading location objects from an Excel file, and construct location objects. I need to set the TimeZone property of the location. There's a timezone table, and the timezoneid is set as the foreign key in the location table.

 TimeZone tz = LocationsRepo.GetTimeZoneByName(timezone);

 if (tz != null)
 {
      currentLocation.TimeZone = tz;
 }

Now when I try to save the location as follows:

  // Check if an entirely new entity is added or an existing entity is being updated.
            Location existingLocation = locations.SingleOrDefault(
                    l =>
                        l.LocationCode.Equals(location.LocationCode, StringComparison.OrdinalIgnoreCase));

            if (existingLocation != null)
            {
                location.LocationId = existingLocation.LocationId;
                ClientContext.Entry(existingLocation).State =
                              ClientContext.Entry(existingLocation).State == System.Data.Entity.EntityState.Added
                                       ? System.Data.Entity.EntityState.Detached
                                       : System.Data.Entity.EntityState.Deleted;
            }
   ClientContext.Entry(location).State = System.Data.Entity.EntityState.Added;


        return ClientContext.SaveChanges();

I get the error that I'm trying to insert a duplicate key in the timezone table. I understand this is because the ClientContext doesn't recognize the location.Timezone because it was set outside the ClientContext. So I tried attaching it:

ClientContext.TimeZones.Attach(location.TimeZone);
ClientContext.SaveChanges();

Then I get the error that I'm trying to insert a duplication location object. Because this timezone is already set to a location existing in the database, and it's trying to attach that.

What's the workaround for this?

来源：https://stackoverflow.com/questions/37202341/ef-associated-entity-is-trying-to-add-to-the-database-when-adding-an-entity