How to retrieve specific columns in Entity Framework

Question

Can I make my EF objects retrieve only specific columns in the sql executed?

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), Name(int), Data(blob), how can I make my query be

select Id, Name from TableName

instead of

select Id, Name, Data from TableName

From the suggestion below, my method is

 public List<T> GetBy<T>(DbContext context,Expression<Func<T, bool>> exp, Expression<Func<T,T>> columns) where T : class
    {

        return dbContext.Set<T>().Where(exp).Select<T,T>(columns).ToList();
    }

And I'm calling it like so

List<CampaignWorkType> list = GetBy<CampaignWorkType>(dbContext, c => c.Active == true, n => new { n.Id, n.Name });

i got an error like below.

Cannot implicitly convert type 'AnonymousType#1' to 'Domain.Campaign.CampaignWorkType'

how i can solve this?

Answer 1

The solution is:

First, define a surrogate type:

public class CampaignWorkTypesSimpleList
{
   public int Id { get; set; }
   public string Name { get; set; }
}

Then change generic method like this:

public List<U> GetBy<T,U>(DbContext context,Expression<Func<T, bool>> exp, Expression<Func<T,U>> columns) 
                where T : class
                where U : class
{

  return dbContext.Set<T>().Where(exp).Select<T, U>(columns).ToList();
}

Finally, execute it.

List<CampaignWorkTypesSimpleList> list = this.GetBy<CampaignWorkType, CampaignWorkTypesSimpleList>(dbContext, c => c.Active == true, n => new CampaignWorkTypesSimpleList { Id = n.Id, Name = n.Name });

Answer 2

假设您正在使用EF设计器，请使用实体模型设计图从实体模型中删除[数据]列。