Entity Framework Code First IQueryable

问题

I am using Entity Framework Code First and ran into a small road block. I have a class "Person" defined as such:

public class Person
{
    public Guid Id { get; set; }
    public virtual ICollection<History> History { get; set; }
}

and a class "History" defined as such:

public class History
{
    public Guid Id { get; set; }
    public virtual Person Owner { get; set; }
    public DateTime OnDate { get; set; }
}

However, when I call:

IEnumerable<History> results = person.History
                               .OrderBy(h => h.OnDate)
                               .Take(50)
                               .ToArray();

It appears to pull all of the history for the person, then order it and such in memory. Any recommendations on what I'm missing?

Thanks in advance!

回答1:

Because you are querying an IEnumerable (ie: LINQ to Objects) not IQueryable (ie: LINQ to Entities) given by EF.

Instead you should use

IEnumerable<History> results = context.History.Where(h => h.Person.Id = "sfssd").OrderBy(h => h.OnDate).Take(50)

回答2:

This question and the accepted answer are both a bit old. Code like this would, as the original question points, load the entire history for the person from the database - not good!

var results = person
    .History
    .OrderBy(h => h.OnDate)
    .Take(50)
    .ToArray();

With EF 6 there is an easy solution. Without rearranging your query, you can have it work the IQueryable way by making use of the DbContext.Entry method, the DbEntryEntity.Collection method, and the DbCollectionEntry.Query method.

var results = dbContext
    .Entry(person)
    .Collection(p => p.History)
    .Query()
    .OrderBy(h => h.OnDate)
    .Take(50)
    .ToArray();

来源：https://stackoverflow.com/questions/6470278/entity-framework-code-first-iqueryable

标签

entity-framework-4.1