How do I create a nested group-by dictionary using LINQ? - linq

Question

How would I create a nested grouping for the table below, using LINQ? I want to group by Code, then by Mktcode.
Code Mktcode Id
==== ======= ====
1 10 0001
2 20 0010
1 10 0012
1 20 0010
1 20 0014
2 20 0001
2 30 0002
1 30 0002
1 30 0005
I'd like a dictionary, in the end, like
Dictionary<Code, List<Dictionary<Mktcode, List<Id>>>>
So the values of this dictionary would be
{1, ({10,(0001,0012)}, {20,(0010,0014)}, {30, (0002, 0005)})},
{2, ({20,(0001, 0010)}, {30, (0020)} )}

Answer 1

I'd think of it this way:
You're primarily grouping by code, so do that first
For each group, you've still got a list of results - so apply another grouping there.
Something like:
var groupedByCode = source.GroupBy(x => x.Code);
var groupedByCodeAndThenId = groupedByCode.Select(group =>
new { Key=group.Key, NestedGroup = group.ToLookup
(result => result.MktCode, result => result.Id));
var dictionary = groupedByCodeAndThenId.ToDictionary
(result => result.Key, result => result.NestedGroup);
That will give you a Dictionary<Code, Lookup<MktCode, Id>> - I think that's what you want. It's completely untested though.

Answer 2

You can build lookups (Kinds of Dictionary<,List<>>) using group by into
var lines = new []
{
new {Code = 1, MktCode = 10, Id = 1},
new {Code = 2, MktCode = 20, Id = 10},
new {Code = 1, MktCode = 10, Id = 12},
new {Code = 1, MktCode = 20, Id = 10},
new {Code = 1, MktCode = 20, Id = 14},
new {Code = 2, MktCode = 20, Id = 1},
new {Code = 2, MktCode = 30, Id = 2},
new {Code = 1, MktCode = 30, Id = 2},
new {Code = 1, MktCode = 30, Id = 5},
};
var groups = from line in lines
group line by line.Code
into codeGroup
select new
{
Code = codeGroup.Key,
Items = from l in codeGroup
group l by l.MktCode into mktCodeGroup
select new
{
MktCode = mktCodeGroup.Key,
Ids = from mktLine in mktCodeGroup
select mktLine.Id
}
};

Answer 3

Here's how I'd do it:
Dictionary<Code, Dictionary<MktCode, List<Id>>> myStructure =
myList
.GroupBy(e => e.Code)
.ToDictionary(
g => g.Key,
g => g
.GroupBy(e => e.Mktcode)
.ToDictionary(
g2 => g2.Key,
g2 => g2.Select(e => e.Id).ToList()
)
)
Here's the breakdown of the process:
Group the elements by Code, and create an outer dictionary where the key is that code.
myList
.GroupBy(e => e.Code)
.ToDictionary(
g => g.Key,
For each of the keys in the outer dictionary, regroup the elements by Mktcode and create an inner dictionary.
g => g
.GroupBy(e => e.Mktcode)
.ToDictionary(
g2 => g2.Key,
For each of the keys in the inner dictionary, project the id of those elements and convert that to a list.
g2 => g2.Select(e => e.Id).ToList()
)
)