Only get specific columns

entity-framework

Question

Can I make my EF objects retrieve only specific columns in the sql executed? If I am executing the below code to retrieve objects, is there something I can do to only get certain columns if wanted?

public IEnumerable<T> GetBy(Expression<Func<T, bool>> exp)
{
    return _ctx.CreateQuery<T>(typeof(T).Name).Where<T>(exp);
}

This would generate a select clause that contains all columns. But, if I have a column that contains a large amount of data that really slows down the query, how can I have my objects exclude that column from the sql generated?

If my table has Id(int), Status(int), Data(blob), how can I make my query be

select Id, Status from TableName

instead of

select Id, Status, Data from TableName

From the suggestion below, my method is

public IEnumerable<T> GetBy(Expression<Func<T, bool>> exp, Expression<Func<T, T>> columns)
{
    return Table.Where<T>(exp).Select<T, T>(columns);
}

And I'm calling it like so

mgr.GetBy(f => f.Id < 10000, n => new {n.Id, n.Status});

However, I'm getting a compile error:

Cannot implicitly convert type 'AnonymousType#1' to 'Entities.BatchRequest'

Accepted Answer

Sure. Projection does this:

var q = from r in Context.TableName
        select new 
        {
            Id = r.Id,
            Status = r.Status
        }

Here's an actual example (obviously, my DB has different tables than yours). I added my EF model to LINQPad and typed the following query:

from at in AddressTypes
select new
{
    Id = at.Id,
    Code = at.Code
}

LINQPad shows me that the generated SQL is:

SELECT 
    1 AS [C1], 
    [Extent1].[AddressTypeId] AS [AddressTypeId], 
    [Extent1].[Code] AS [Code]
FROM 
    [dbo].[AddressType] AS [Extent1]

None of the other fields from the table are included.

Responding to updated question

Your columns argument says it takes a type T and returns the same type. Therefore, the expression you pass must conform to this, or you need to change the type of the argument, i.e.:

public IEnumerable<U> GetBy<U>(Expression<Func<T, bool>> exp, Expression<Func<T, U>> columns)
{
    return Table.Where<T>(exp).Select<T, U>(columns);
}

Now the expression can return any type you care to use.



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