Select Single Parent with Multiple Children into Custom Entity

Question

I am trying to produce a custom entity using a select statement that involves tables a parent/child relation (1:n). I made an attempt to group the objects selected, but I am having trouble coming up with a solution.

My data would be this format:

ParentTable
Userid | Col1 | Col2 | ... | Coln

ChildTable:
Id | Userid | Col1 | Col2 | ... | Coln

The result object I am trying to produce would be this:

Custom { UserID, IEnumerable<ChildTable> }

I have written a query:

    from parent in db.ParentTable
    join child in db.ChildTable on new { Userid = parent.Id } equals new { Userid = child.Userid } into child_join
    from child in child_join.DefaultIfEmpty()
    where
         child.Col1 == Argument1 &&
         child.Col2 == Argument2
    select new {
         Username = parent.Username,
         child.Col1,
         child.Col2,
         child.Col3,
         child.Coln 
   }

That will return this result:

Username | Child Col1 | Child Col2 | Child Col3 | .. |Child Coln
asdf        1              11           22        ..     33
asdf        2              22           33        ..     44
asdf        3              33           44        ..     55
qwer        4              44           55        ..     66
qwer        5              55           66        ..     77
qwer        6              66           77        ..     88
zxcv        7              77           88        ..     99
zxcv        8              88           99        ..     00

I would like to achieve this:

Username | IEnumerable<Child>
asdf     |     {{1             11           22        ..     33}
         |      {2             22           33        ..     44}
         |      {3             33           44        ..     55}}
qwer     |     {{4             44           55        ..     66}
         |      {5             55           66        ..     77}
         |      {6             66           77        ..     88}}
zxcv     |     {{7             77           88        ..     99}
                {8             88           99        ..     00}}

I am trying to group the items by the username and produce a custom object in the form of Custom { UserID, IEnumerable } where I can serialize the objects once completed. Any help is appreciated. Edit: Regarding the data structure, it can not be changed as I am connecting to a 3rd party system so I am working with what is given.

Answer 1

Although this is not an ideal solution, I decided to group the object after I had executed the query, but prior to serializing the data. I tried to apply a 'group by' clause, but I do not have any aggregates to apply as I was not attempting to find a max or sum of the child items, rather a straight group. My (unrefined) solution looks like so, using my original query, but placed into a custom entity class:

from parent in db.ParentTable
join child in db.ChildTable on new { Userid = parent.Id } equals new { Userid = child.Userid }
where
      child.Col1 == Argument1 &&
      child.Col2 == Argument2
select new ChildDataDomainModelClass {
      Username = parent.Username,
      child.Col1,
      child.Col2,
      child.Col3,
      child.Coln 
}

 //I had grouped the object like this, where the object type returned on the grouping is of type IEnumerable<IGrouping<KeyValuePair<int, string>, ChildDataDomainModelClass>>:

List<ResultDataClass> returnData= new List<ResultDataClass>();
var groupedByUserName = dataResult.GroupBy(item => new KeyValuePair<int, string>(item.UserID, item.UserName), item => item);
        foreach (var userData in groupedByUserName)
        {
             var newdata = new ResultDataClass(userData.Key.Key, userData.Key.Value, userData.Select(item => item.TrackData)); //TrackData is what I have named the Child Data that was returned
             returnData.Add(newdata);
        }

I do appreciate the input offered. If anyone has a more elegant solution to this problem where I do not have to loop over the objects prior to creating the custom class, let me know. Ideally, I would not have to create the 'ChildDataDomainModelClass' and through my query, I would select new 'ResultDataClass'. I hope this would help others that have encountered this problem. If I come up with a solution, I will update this post accordingly. I appreciate everyone's input. Thanks.

Answer 2

I think maybe what you are looking for here is a subquery. I would try something like this:

from parent in db.ParentTable
select new {
  Username = parent.Username,
  Children = (from child in db.ChildTable
              where child.UserId == parent.UserId
              select child)
}

If you just want to populate your list from your groupby query without looping you can do:

List<ResultDataClass> returnData = dataResult.GroupBy(item => new KeyValuePair<int, string>(item.UserID, item.UserName), item => item)
                            .Select(rdc => var newdata = new ResultDataClass(userData.Key.Key, userData.Key.Value, userData.Select(item => item.TrackData))).ToList();