Entity Framework How to properly Update an entity Code First?

Question

I have the following update method in my generic Repository

public class Repository<T> : IRepository<T> where T : class
{
    private readonly DbSet<T> _dbSet;
    public virtual T Update(T item) {
        return _dbSet.Attach(item);
    }
}

the UnitOfWork has a commit method which calls the SaveChanges on the context. More details here
https://codereview.stackexchange.com/questions/19037/entity-framework-generic-repository-pattern

When I update an entity and then call

ProductRepository.Update(modifiedProduct);
UnitOfWork.Commit;

Nothing floats down to the database.

However , Merely calling the Commit works ( no call to the update method ).

So, what is the Attach Method doing that causes the changes to not flow down to the database. I think the attach call is the correct call to make in the Update Method. So, what is causing the unexpected behavior.

From the EF Source code on CodePlex

/// <summary>
///     Attaches the given entity to the context underlying the set.  That is, the entity is placed
///     into the context in the Unchanged state, just as if it had been read from the database.
/// </summary>
/// <param name="entity"> The entity to attach. </param>
/// <returns> The entity. </returns>
/// <remarks>
///     Attach is used to repopulate a context with an entity that is known to already exist in the database.
///     SaveChanges will therefore not attempt to insert an attached entity into the database because
///     it is assumed to already be there.
///     Note that entities that are already in the context in some other state will have their state set
///     to Unchanged.  Attach is a no-op if the entity is already in the context in the Unchanged state.
/// </remarks>
public object Attach(object entity)
{
    Check.NotNull(entity, "entity");

    InternalSet.Attach(entity);
    return entity;
}

devdigital · Accepted Answer · 2012-12-19 09:33:17Z

5

///     Attach is used to repopulate a context with an entity that is known to already exist in the database.
///     SaveChanges will therefore not attempt to insert an attached entity into the database because
///     it is assumed to already be there.
///     Note that entities that are already in the context in some other state will have their state set
///     to Unchanged.

After attaching the entity, the state will be Unchanged, so no UPDATE sql is going to fire for that entity. You would need to manually set the state of the entity after attaching.

answered Dec 19, 2012 at 9:33

devdigital

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3 Comments

ashutosh raina Over a year ago

So I need to set the EntityState as Modified.. but My problem is that currently my Repository knows nothing about the Context. Anyway around it ?

Jayantha Lal Sirisena Over a year ago

I think it's better to have in the repository level. If not you are not using (hidden) some capabilities of EF.

devdigital Over a year ago

Yes your repository should have a reference to the context, it can be a dependency if you wish to share a context across repositories

Jayantha Lal Sirisena · Accepted Answer · 2012-12-19 11:14:27Z

4

You need to attach the object as modified or added to produce a query to database.

  public virtual T AttachAsModified(T item) {
        item = _dbSet.Attach(item);
        db.Entry(item).State = System.Data.EntityState.Modified
        return item;
    }

edited Dec 19, 2012 at 11:14

answered Dec 19, 2012 at 9:34

Jayantha Lal Sirisena

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2 Comments

ashutosh raina Over a year ago

public virtual T AttachAsModified(T item) { db.Entry(item).State = System.Data.EntityState.Modified } works... however adding the _dbset.Attach(item) causes the problem to prop up.. Have I understood the Attach method incorrectly ?

ashutosh raina Over a year ago

The state needs to be set after we Attach the entity.

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