如何检索实体框架特定的列 [英] How to retrieve specific columns in Entity Framework
问题描述
我可以让我的EF对象检索在SQL只有特定的列执行?
如果我有一列包含大量数据的真正减慢查询,哪能有我的对象排除了该列从生成的SQL?
如果我的表中有ID(INT),姓名(INT),数据(BLOB),我怎样才能使我的查询是
SELECT标识,从表名名称
而不是
选择编号,名称,从表名数据
从下面的建议下,我的方法是
公开名单< T> GetBy< T>(的DbContext的背景下,前pression< Func键< T,布尔>>精通,防爆pression< Func键< T,T>>列)其中T:类
{
返回dbContext.Set< T>(),其中(EXP)。选择< T,T>(列).ToList();
}
和我打电话,象这样
名单,其中,CampaignWorkType>名单= GetBy< CampaignWorkType>(的DbContext,C => c.Active == true,则N =>新建{n.Id,n.Name});
我得到了一个错误,如下面的。
无法隐式转换类型'AnonymousType#1到Domain.Campaign.CampaignWorkType
我怎么能解决这个问题?
解决的办法是:
首先,定义一个代理类型:
公共类CampaignWorkTypesSimpleList
{
公众诠释编号{获得;组; }
公共字符串名称{;组; }
}
然后改变通用的方法是这样的:
公开名单< U> GetBy< T,U>(的DbContext的背景下,前pression< Func键< T,布尔>>精通,防爆pression< Func键< T,U>>列)
其中T:类
其中U:类
{
返回dbContext.Set< T>(),其中(EXP)。选择< T,U>(列).ToList();
}
最后,执行它。
名单,其中,CampaignWorkTypesSimpleList>名单= this.GetBy< CampaignWorkType,CampaignWorkTypesSimpleList>(的DbContext,C => c.Active == true,则N =>新建CampaignWorkTypesSimpleList {n = n.Id,名称= n.Name});
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?
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 });
这篇关于如何检索实体框架特定的列的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,也希望大家多多支持IT屋!