如何通过动态项目对linq查询进行分组

Question

I have a grouped linq query that groups items by category, and then the location within that category:

var categories = db.Items.GroupBy(p => p.Category).
    Select(group =>
    new
    {
        name = group.Key,
        data = new List<int>()
        {
            group.Count(p => p.City == "Sydney"),
            group.Count(p => p.City == "Melbourne"),
            group.Count(p => p.City == "Brisbane"),
            group.Count(p => p.City == "Perth"),
            group.Count(p => p.City == "Canberra"),
        },
        total = group.Count()
    });

As you can see, I'm hardcoding the location to get the count for each location within a category. I don't want to hardcode this as the locations are stored separately in the database, is there a way I can modify the query so the counts are dynamic? For example, for everything in db.locations?

Answer 1

You can use another GroupBy.

var categories = db.Items.GroupBy(p => p.Category).Select(group =>
new
{
    Name = group.Key,
    Data = group.GroupBy(g => g.City),
    Total = group.Count()
});

You can also have another anonymous type inside.

var categories = db.Items.GroupBy(p => p.Category).Select(group =>
new
{
    Name = group.Key,
    Data = group.GroupBy(g => g.City).Select(c => 
           new
           {
               CityName = c.Key, 
               Count = c.Count()
           }),
    Total = group.Count()
});

Here is how to use it. if you are confused.

foreach(var category in categories)
{
    var name = category.Name;
    var data = category.Data;
    var total = category.Total;

    foreach(var location in data)
    {
        var cityname = location.CityName;
        var count = location.Count;
    }
}

Note that if you want to pass this query as parameter to somewhere else, you have to use dynamic keyword instead of var .

var would be fast. but dynamic is slow so in that case I recommend you to write custom class for holding your data instead of using anonymous type.

Answer 2

var categories = db.Items.GroupBy(p => p.Category).
Select(group =>
new
{
    name = group.Key,
    data = new List<int>()
    {
        group.Count(p => p.City == db.locations.select(loc=>loc.ToString()).ToString()),
    },
    total = group.Count()
});