Using EF to assign the results of a LINQ query to another object

Question

I am not sure exactly how to word the Title so I would also appreciate any thoughts on that part and will update this accordingly.

Using C#, standard console program.

I have a database object with a number of fields, any or all of them may be being changed. I have another object with the same fields that contains the new data. I can go through one by one assigning them and it work, but if i just do the entire thing at once it does not work.

Probably clearer with some code. the code that DOES work first: I have all of the data that I need updated in newStop.

I go out and get the data from the database and store it in findStop2. I then can go through all the fields one by one (only one is shown here) and assign the values from newStop to findStop2. When I do a db.SaveChanges everything works.

 var findStop2 = (from s in db.stop_details 
         where s.customer_id == customer_id && s.stop_id == newStop.stop_id 
         select s)
     .First();

 //if it did not error out it found it
 findStop2.ship_date = newStop.ship_date;

But there are a lot of fields. Why can I not just use:

findStop2 = newStop;

When I do this and do a SaveChanges there is no error thrown but the database also is not updated.

Solution -- Final changed code that works:

var findStop2 = (from s in db.stop_details where
                 s.customer_id == customer_id && s.stop_id == newStop.stop_id
                                 select s).First();
                //if it did not error out it found it
                newStop.id = findStop2.id;
                db.Entry(findStop2).CurrentValues.SetValues(newStop);

Answer 1

If all your values are scalar, you can use (in EF6):

db.Entry(findStop2).CurrentValues.SetValues(newStop);

This will correctly track changes and everything

Answer 2

If I understood your question right, you need to cast one object to another. To do this, you can write:

var myObject = db.stop_details
    .Where(sd => sd.customer_id == customer_id && sd.stop_id == newStop.stop_id)
    .Select(entity => new MyObject 
    {
        // Your properties here
    })
    .First();

Alternatively, if you perform such kind of operations a lot of time (and I guess you do) you can use some mapping libraries, for example AutoMapper. If you are using mapper all that you need is to write all possible configurations of how to map one object of concrete type to another and vice versa. The in all places you just need to call something like

var myObject = Mapper.Map<OutObjectType>(inputObject)

and that it. Mapper will return to you newly created object with all fields set up.

P.S. If you decide to use Automapper it works very good with inheritance, interfaces and other stuff which can be tricky to configure

Answer 3

The reason why you can't just assign an in-memory object to one retrieved from Entity Framework and then save it to the database is that you lose the hidden state that EF adds to all of the objects it creates. So after your findStop2 = newStop;, findStop2 is no longer tracked by your DB Context.

What you can do is set the state on your object directly before saving your changes.

context.Entry(newStop).State = EntityState.Modified;

Be careful when you do this as it will overwrite everything in the database to the properties on your newStop object. (You will have to set the ID, or else it will add another record for example.)

EDIT

As Jcl mentioned in the comics, you will need to do something with findStop2 as it is also being tracked and will therefore create a separate record in the database anyway. If you don't need any properties from it at all, you don't need to retrieve it from the database.

You can either call a context.Stops.Remove(findStop2), or do something similar to the above:

context.Entry(findStop2).State = EntityState.Detached;