EF builds EntityCollection, but I (think I) want IQueryable

问题

I have an entity A with a simple navigation property B. For any given instance of A, we expect several related thousand instances of B.

There is no case where I call something like:

foreach(var x in A.B) { ... }

Instead, I'm only interested in doing aggregate operations such as

var statY = A.B.Where(o => o.Property == "Y");
var statZ = A.B.Where(o => o.CreateDate > DateTime.Now.AddDays(-1));

As far as I can tell, EF instantiates thousands of references to B and does these operations in memory. This is because navigation properties use EntityCollection. Instead, I'd like it to perform these queries at the SQL level if possible.

My current hunch is that Navigation Properties may not be the right way to go. I'm not attached to EF, so I am open to other approaches. But I'd be very interested to know the right way to do this under EF if possible.

(I'm using EF4.)

回答1:

CreateSourceQuery seems to do the trick.

So my examples would now be:

var statY = A.B.CreateSourceQuery().Where(o => o.Property == "Y");
var statZ = A.B.CreateSourceQuery().Where(o => o.CreateDate > DateTime.Now.AddDays(-1));

回答2:

There's one thing you should know. Members that derives from IQueryable<> are executed on the server, not in memory. Members which are derived from IEnumerable<> is executed in memory. for example

var someEntities = db.SomeEntities; <-- returns an IQueryable<> object. no data fetched. SomeEntities table may contain thousands of rows, but we are not fetching it yet, we are just building a query.
someEntities = someEntities.Where(s => s.Id > 100 && s.Id < 200); <-- creates expression tree with where statement. The query is not executed yet and data is not fetched on the client. We just tell EF to perform a where filter when query will execute. This statement too returns an IQueryable<> object.
var entities = someEntities.AsEnumerable(); <-- here we tell EF to execute query. now entities will be fetched and any additional linq query will be performed in memory.

you can also fetch the data using foreach, calling ToArray() or ToList<>.

Hope you understand what I mean, and sorry for my english :)

来源：https://stackoverflow.com/questions/4317879/ef-builds-entitycollection-but-i-think-i-want-iqueryable

标签

entity-framework

iqueryable