DbSet doesn't have a Find method in EF7

Question

I am trying to create a generic repository to access my database. In EF6 I was able to do that in order to get a specific entity:

protected IDbSet d         


        

Answer 1

Let me contribute a revision that includes building the expression. I'll confess I didn't actually test this ;-)

    public static TEntity Find<TEntity>(this DbSet<TEntity> dbSet, params object[] keyValues) where TEntity : class
    {
        // Find DbContext, entity type, and primary key.
        var context = ((IInfrastructure<IServiceProvider>)dbSet).GetService<DbContext>();
        var entityType = context.Model.FindEntityType(typeof(TEntity));
        var key = entityType.FindPrimaryKey();

        // Build the lambda expression for the query: (TEntity entity) => AND( entity.keyProperty[i] == keyValues[i])
        var entityParameter = Expression.Parameter(typeof(TEntity), "entity");
        Expression whereClause = Expression.Constant(true, typeof(bool));
        uint i = 0;

        foreach (var keyProperty in key.Properties) {
            var keyMatch = Expression.Equal(
                Expression.Property(entityParameter, keyProperty.Name),
                Expression.Constant(keyValues[i++])
            );

            whereClause = Expression.And(whereClause, keyMatch);
        }

        var lambdaExpression = (Expression<Func<TEntity,bool>>)Expression.Lambda(whereClause, entityParameter);

        // Execute against the in-memory entities, which we get from ChangeTracker (but not filtering the state of the entities).
        var entries = context.ChangeTracker.Entries<TEntity>().Select((EntityEntry e) => (TEntity)e.Entity);
        TEntity entity = entries.AsQueryable().Where(lambdaExpression).First(); // First is what triggers the query execution.

        // If found in memory then we're done.
        if (entity != null) { return entity; }

        // Otherwise execute the query against the database.
        return dbSet.Where(lambdaExpression).First();
    }

Answer 2

I've taken some of the previously provided answers and tweaked them to fix a couple of problems:

• Implicitly captured closure

• Key shouldn't be hard coded to "Id"

  public static TEntity Find<TEntity>(this DbSet<TEntity> set, params object[] keyValues) where TEntity : class
  {
      var context = set.GetService<DbContext>();
  
      var entityType = context.Model.FindEntityType(typeof(TEntity));
      var key = entityType.FindPrimaryKey();
  
      var entries = context.ChangeTracker.Entries<TEntity>();
  
      var i = 0;
      foreach (var property in key.Properties)
      {
          var i1 = i;
          entries = entries.Where(e => e.Property(property.Name).CurrentValue == keyValues[i1]);
          i++;
      }
  
      var entry = entries.FirstOrDefault();
      if (entry != null)
      {
          // Return the local object if it exists.
          return entry.Entity;
      }
  
      var parameter = Expression.Parameter(typeof(TEntity), "x");
      var query = set.AsQueryable();
      i = 0;
      foreach (var property in key.Properties)
      {
          var i1 = i;
          query = query.Where((Expression<Func<TEntity, bool>>)
           Expression.Lambda(
               Expression.Equal(
                   Expression.Property(parameter, property.Name),
                   Expression.Constant(keyValues[i1])),
               parameter));
          i++;
      }
  
      // Look in the database
      return query.FirstOrDefault();
  }
  

Answer 3

Can't comment because of reputation, but if you use RC2 (or later?) you should use

var context = set.GetService<ICurrentDbContext>().Context;

instead of

var context = set.GetService<DbContext>();

Answer 4

An edit was proposed to change ".First()" to ".FirstOrDefault()" in the very last line of my earlier post. The edit was voted down, but I agree with it. I would expect the function to return null if the key was not found. I would not want it to throw an exception. In most cases I would want to know if the key existed in the set, and handling an exception is a very slow way of figuring that out.

Answer 5

here is what I use. Not a find method, but works like a charm

var professionalf = from m in _context.Professionals select m;
professionalf = professionalf.Where(s => s.ProfessionalId == id);
Professional professional = professionalf.First();

Answer 6

Here's a very crude, incomplete, and untested implementation of .Find() as an extension method. If nothing else, it should get you pointed in the right direction.

The real implementation is tracked by #797.

static TEntity Find<TEntity>(this DbSet<TEntity> set, params object[] keyValues)
    where TEntity : class
{
    var context = ((IAccessor<IServiceProvider>)set).Service.GetService<DbContext>();

    var entityType = context.Model.GetEntityType(typeof(TEntity));
    var key = entityType.GetPrimaryKey();

    var entries = context.ChangeTracker.Entries<TEntity>();

    var i = 0;
    foreach (var property in key.Properties)
    {
        var keyValue = keyValues[i];
        entries = entries.Where(e => e.Property(property.Name).CurrentValue == keyValue);
        i++;
    }

    var entry = entries.FirstOrDefault();
    if (entry != null)
    {
        // Return the local object if it exists.
        return entry.Entity;
    }

    // TODO: Build the real LINQ Expression
    // set.Where(x => x.Id == keyValues[0]);
    var parameter = Expression.Parameter(typeof(TEntity), "x");
    var query = set.Where((Expression<Func<TEntity, bool>>)
        Expression.Lambda(
            Expression.Equal(
                Expression.Property(parameter, "Id"),
                Expression.Constant(keyValues[0])),
            parameter));

    // Look in the database
    return query.FirstOrDefault();
}