如何按子实体的属性分组并获取 linq 中子实体的总和？

Question

I need to group data by child entity and get sum of child entity. So, I tried this linq:

context.Parent.Include(p => p.Child).GroupBy(p => p.Child.Type).Select(g => g.Sum(p => p.Child.Amount));

Above linq results in an error. However, if I were to get sum of parent entity it will work.

context.Parent.Include(p => p.Child).GroupBy(p => p.Child.Type).Select(g => g.Sum(p => p.Amount));

Why cannot I get the sum of child entity?

Answer 1

It is limitation. After GroupBy you cannot use navigation properties. Also after GroupBy all Includes are completely ignored, so omit them.

You query can be written in the following way:

var query = 
    from p in context.Parent
    group p.Child by p.Child.Type into g
    select new 
    {
        Type = g.Key,
        Sum = g.Sum(c => c.Amount)
    };

Lambda syntax variant:

var query = context.Parent
    .GroupBy(p => p.Child.Type, p => p.Child)
    .Select(g => new 
    {
       Type = g.Key, 
       Sum = g.Sum(c => c.Amount)
    });

Answer 2

Apparently, in your model you have a sequence of Parents . Every Parent has a property Child . Every Child has two properties: Amount and Type .

You forgot to tell us your specification, and gave us some code that does not do what you want. So it is a bit difficult for us to know what you want.

I think that you want to make groups of Parents that have the same value for Parent.Child.Type . From every Parent in the group you want to take the value of Parent.Child.Amount and sum theses values.

You are right, for this you should use one of the overloads of Enumerable.GroupBy. My advice would be to use the overload that has a parameter resultSelector:

// make groups of Parents with same value of Parents.Child.Type:
var result = dbContext.Parents.GroupBy(parent => parent.Child.Type,

// parameter resultSelector: for every Type, and all Parents that have this Child.Type
// make one new:
(type, parentsWithThisChildType) => new
{
    // do you want to know the type?
    ChildType = type,

    // from all parents in this group, select the Parent.Child.Amount and Sum them:
    TotalChildAmounts = parentsWithThisChildType
        .Select(parent => parent.Child.Amount)
        .Sum(),
})

In words: from your sequence of Parents make groups of Parents that have the same value for Parent.Child.Type. From every group make one object as follows:

• Is desired, take the common Type of all Parent.Child.Type in this group
• from every Parent in the group, select the value of Parent.Child.Amount.
• Sum all values of this Amound, and put the result in TotalChildAmounts