How creating nested hash tables explicitly/imperitively was much simpler than LINQ expressions

somecode.cs

// This is the code that replaced all the code below this function:
protected override IDictionary<int, Dictionary<string, string>> GroupAndFlatten(IEnumerable<PassengerImport> imports) {
  var paxImports = imports.GroupBy(x => x.PassengerId);
  var paxes = new Dictionary<int,Dictionary<string,string>>();
  foreach(var row in imports){
    var paxid = row.PassengerId;
    if (!paxes.ContainsKey(paxid)) paxes[paxid] = new Dictionary<string,string>();
    var paxHash = paxes[paxid];
    if(paxHash.ContainsKey(row.Key)) paxHash[row.Key] += " "+ row.Value;
    else paxHash[row.Key] = row.Value;
  }
  return paxes;
}

protected override IEnumerable<IDictionary<string, object>> Convert(IEnumerable<PassengerImport> imports) {
            // resolve any laziness, we need to loop over this multiple times
            foreach(var paxHash in GroupAndFlatten(imports)){
                var o  =new Dictionary<string, object>(){
                    {"PaxId", paxHash.Key},
                    {"SpecialNeed", paxHash.Value.Get("PROG_NUM","")+" "+paxHash.Value.Get("PROG_NAME","")}
                };
                yield return o;
            }
        }
    }


// This is the code I couldnt understand
object MergeFields(IGrouping<string, object> x) {
  // don't want to merge this key
  if (x.Key == this.PassengerIdParamName) {
    return x.First();
  }
  return (object) string.Join(" ", x);
}

IDictionary<string, object> CombineRows(ICollection<PassengerImport> imps, IDictionary<int, List<int?>> importLines) {
  return importLines
    // convert to paxId / lineNumber pairs
    .SelectMany(x => x.Value.Select(y => new { pId = x.Key, ln = y }))
    // `IEnumerable<PassengerImport>`s for each paxId / lineNumber
    .Select(x => imps.Where(y => y.PassengerId == x.pId &&
                            y.LineNumber == x.ln))
    // convert those to lists of SQL params for SP calls
    .Select(base.Convert)
    // expect only one param set
    .Select(x => x.Single())
    // get a list of all key/value pairs
    .SelectMany(x => x)
    // group by key
    .GroupBy(x => x.Key, x => x.Value)
    // join values
    .ToDictionary(x => x.Key, MergeFields);
}

protected override IEnumerable<IDictionary<string, object>> Convert(IEnumerable<PassengerImport> imports) {
  // resolve any laziness, we need to loop over this multiple times
  var imps = imports.ToList();
  // get a dict mappping: paxId -> list of unique line numbers
  var importLines = imps
    .GroupBy(x => x.PassengerId)
    .ToDictionary(x => x.Key, x => x.Select(y => y.LineNumber).Distinct().ToList());

  var hasMultipleSpecNeeds = importLines.Any(x => x.Value.Count > 1);                

  if (hasMultipleSpecNeeds) {
    return new[] { CombineRows(imps, importLines) };
  } else {
    return base.Convert(imports);
  }
}
}