只得到具体的列 [英] Only get specific columns
问题描述
我可以让我的EF对象只检索执行的sql中的特定列吗?如果我正在执行以下代码来检索对象,有没有什么可以做的,只有在需要时才能获得某些列?
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);
}
这将生成一个包含所有列的select子句。但是,如果我有一个列包含大量的数据真的会减慢查询速度,那么我怎么能让我的对象从sql生成的列中排除该列?
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?
如果我的表有Id(int),Status(int),Data(blob),我如何使我的查询是
If my table has Id(int), Status(int), Data(blob), how can I make my query be
select Id, Status from TableName
而不是
select Id, Status, Data from TableName
从我的方法是
public IEnumerable<T> GetBy(Expression<Func<T, bool>> exp, Expression<Func<T, T>> columns)
{
return Table.Where<T>(exp).Select<T, T>(columns);
}
我打电话就这样
mgr.GetBy(f => f.Id < 10000, n => new {n.Id, n.Status});
但是,我收到一个编译错误:
However, I'm getting a compile error:
不能将类型AnonymousType#1隐式转换为Entities.BatchRequest
Cannot implicitly convert type 'AnonymousType#1' to 'Entities.BatchRequest'
推荐答案
当然可以。这样做:
var q = from r in Context.TableName
select new
{
Id = r.Id,
Status = r.Status
}
这是一个实际的例子(显然,我的数据库有不同的表比你的)。我将我的EF模型添加到LINQPad,并键入以下查询:
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向我显示生成的SQL是:
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]
表中没有其他字段。
响应更新的问题
您的列
参数表示需要一个类型T并返回相同的类型。因此,您传递的表达式必须符合此要求,或者您需要更改参数的类型,即:
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);
}
现在表达式可以返回您要使用的任何类型。
Now the expression can return any type you care to use.
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