Entity Framework and adding a reference / entry to a lookup table

Question

I have a project where I use a lookup table to link two tables together:

Day         WeatherLookUp       Weather
---         -------------       -------    
ID (PK) --> DayID            |- ID (PK)
            WeatherID    <---|  Description

This allows me to specify multiple weather conditions for a day.

I can read from this without any issues but my problem is when I come to insert a link between the Day and the Weather table. I created the WeatherLookup table's two columns as the compound primary key of the table and as a result EF doesn't allow me to do a direct insert into the WeatherLookup table.

I had thought I just needed to add a Weather entry like this:

myDay.Weather.Add(new Weather { ID = 2 } );

...but EF thinks I'm trying to add a new weather type.

I'm sure I'm missing something obvious but I can't work out what, do I need to be using Attach() in some way?

Answer 1

You need to attach the Weather entity to the context in order to tell EF that it already exists in the database and doesn't need to be inserted:

var weather = new Weather { ID = 2 };
context.Weather.Attach(weather);
myDay.Weather.Add(weather);

Alternatively you can load the entity from the database (which will attach it implicitly to the context):

var weather = context.Weather.Find(2);
myDay.Weather.Add(weather);

Answer 2

You need to assiociate the "Weather" with the "WeatherLookUp" not only using the ID.

something like this:

Weather weather = new Weather();
weather.Description = "hi";


WeatherLookUp lookup = new WeatherLookUp ();
lookup.Weather = weather;

myDay.Weather.add(Weather);
myDay.WeatherLookUp.add(lookup);
myDay.SaveChanges();