How to use GroupBy properly in LINQ? - linq

Question

I have 4 tables: Post, Category, Relation and Meta
A category can contains multiple posts, and the relation between them is stored in Relation table. A post then can has many extra info that are stored in Meta table. I want to list all post with categories and extra infos, then group them by post's ID.
I have the following query
select p.ID, p.Title, t.Name, m.Key, m.Value from Post p
left join Relation r on p.ID = r.Child
left join Category c on r.Parent = c.ID
left join Meta m on p.ID = m.Object
where m.Type = 'news'
order by p.ID
and with these sample data:
Post
ID Title
1 A
Category
ID Name
1 Tips
2 Tricks
Meta
ID Object Key Value
1 1 Key1 Value 1
2 1 Key2 Value 2
Relation
ID Child Parent
1 1 1
2 1 2
then the result will be
PostID Title Category Key Value
1 A Tips Key1 Value1
1 A Tips Key2 Value2
1 A Tricks Key1 Value1
1 A Tricks Key2 Value2
and I expected the result to be
PostID Title Categories Meta
1 A Tips, Tricks Key1=Value1, Key2=Value2
I wonder if we can convert the query from SQL to LINQ to Entities with EF v4 and the result is stored in a class like this
class Result
{
long ID,
string Title,
List<string> Categories,
Dictionary<string, string> Meta
}
Any helps would be appreciated.

Answer 1

What's the final result you expect from the query
I personally prefer to write the query like
var q = from r in Relation
join p in Post on r.Child equals p.ID
join t in Term on r.Parent equals t.ID
let x = new { p.ID, p.Title, t.Name }
group x by x.ID into g
select g;
this way I think (not sure) the sql generated will be simpler

Answer 2

Now that you're wanting to use EntityFramework, you would merely need to set up you database, edmx with a Result table with an ID and a Title, then Category and Meta tables. Then add one-to-many relationships from the Result table to each the Category and Meta tables.

Answer 3

I'm not 100% sure what you're trying to do, but obviously if you're grouping, the results have to be grouped by anything in the resultset, or be aggregated data. This query will retrieve your results and group by PostId, PostTitle, and CategoryName, generating a single SQL Statement:
var query = from p in Posts
from r in Relations
.Where(r => p.ID == r.Child)
.DefaultIfEmpty()
from c in Categories
.Where(c => r.Parent == c.ID)
.DefaultIfEmpty()
group p by new {ID = p.ID, Title = p.Title, Name = c.Name} into z
select new { ID = z.Key.ID, Title = z.Key.Title, Name = z.Key.Name };
Here is the SQL Generated by this statement:
SELECT [t3].[ID], [t3].[Title], [t3].[value] AS [Name]
FROM (
SELECT [t0].[ID], [t0].[Title], [t2].[Name] AS [value]
FROM [Post] AS [t0]
LEFT OUTER JOIN [Relation] AS [t1] ON [t0].[ID] = [t1].[Child]
LEFT OUTER JOIN [Category] AS [t2] ON [t1].[Parent] = [t2].[ID]
) AS [t3]
GROUP BY [t3].[ID], [t3].[Title], [t3].[value]
Here is the SQL Generated by your original statement:
SELECT [t0].[ID] AS [Key]
FROM [Post] AS [t0]
INNER JOIN [Relation] AS [t1] ON [t0].[ID] = [t1].[Child]
INNER JOIN [Category] AS [t2] ON [t1].[Parent] = [t2].[ID]
GROUP BY [t0].[ID]
GO
-- Region Parameters
DECLARE #x1 Int SET #x1 = 1
-- EndRegion
SELECT [t0].[ID], [t0].[Title], [t2].[Name]
FROM [Post] AS [t0]
INNER JOIN [Relation] AS [t1] ON [t0].[ID] = [t1].[Child]
INNER JOIN [Category] AS [t2] ON [t1].[Parent] = [t2].[ID]
WHERE ((#x1 IS NULL) AND ([t0].[ID] IS NULL)) OR ((#x1 IS NOT NULL) AND ([t0].[ID] IS NOT NULL) AND (#x1 = [t0].[ID]))
GO
-- Region Parameters
DECLARE #x1 Int SET #x1 = 2
-- EndRegion
SELECT [t0].[ID], [t0].[Title], [t2].[Name]
FROM [Post] AS [t0]
INNER JOIN [Relation] AS [t1] ON [t0].[ID] = [t1].[Child]
INNER JOIN [Category] AS [t2] ON [t1].[Parent] = [t2].[ID]
WHERE ((#x1 IS NULL) AND ([t0].[ID] IS NULL)) OR ((#x1 IS NOT NULL) AND ([t0].[ID] IS NOT NULL) AND (#x1 = [t0].[ID]))