Entity Framework ... Four queries into one - entity-framework

Question

I am using EF5 and finding questions as follows:
context.Questions
.OrderBy(x => Guid.NewGuid())
.Where(x => x.Difficulty == difficulty && x.Format == format);
Each Question has a field named duration with values (2, 4, 6 and 8) in minutes.
I have the following int array: { 4, 0, 1, 2 } which means:
Get 4 questions of 2 minutes, 0 of 4 minutes, 1 of 6 minutes and 2 of 8 minutes.
After .OrderBy and .Where I need to get 7 questions as mentioned.
Is there a way to do this without loading all questions or using 4 queries?
Thank You,
Miguel

Answer 1

This is kind of hardcoded - but you get the idea, and solves your problem explicitly. You should be able to convert it to whatever you need from here.
var questions = context.Questions .OrderBy(x => Guid.NewGuid())
.Where(x => x.Difficulty == difficulty && x.Format == format);
var selectedQuestions = questions.Where(q => q.Duration == 2).Take(questionArray[0])
.Union(questions.Where(q => q.Duration == 4).Take(questionArray[1]))
.Union(questions.Where(q => q.Duration == 6).Take(questionArray[2]))
.Union(questions.Where(q => q.Duration == 8).Take(questionArray[3]));
Since you're never enumerating the queryable, EF will do all of these unions in sql, and get all the data in a single call.
Produced SQL:
SELECT [Distinct3].[C1] AS [C1],
[Distinct3].[C2] AS [C2],
[Distinct3].[C3] AS [C3],
[Distinct3].[C4] AS [C4]
FROM (SELECT DISTINCT [UnionAll3].[C1] AS [C1],
[UnionAll3].[C2] AS [C2],
[UnionAll3].[C3] AS [C3],
[UnionAll3].[C4] AS [C4]
FROM (SELECT [Distinct2].[C1] AS [C1],
[Distinct2].[C2] AS [C2],
[Distinct2].[C3] AS [C3],
[Distinct2].[C4] AS [C4]
FROM (SELECT DISTINCT [UnionAll2].[C1] AS [C1],
[UnionAll2].[C2] AS [C2],
[UnionAll2].[C3] AS [C3],
[UnionAll2].[C4] AS [C4]
FROM (SELECT [Distinct1].[C1] AS [C1],
[Distinct1].[C2] AS [C2],
[Distinct1].[C3] AS [C3],
[Distinct1].[C4] AS [C4]
FROM (SELECT DISTINCT [UnionAll1].[Id] AS [C1],
[UnionAll1].[Duration] AS [C2],
[UnionAll1].[Difficulty] AS [C3],
[UnionAll1].[Format] AS [C4]
FROM (SELECT TOP (4) [Project1].[Id] AS [Id],
[Project1].[Duration] AS [Duration],
[Project1].[Difficulty] AS [Difficulty],
[Project1].[Format] AS [Format]
FROM (SELECT NEWID() AS [C1],
[Extent1].[Id] AS [Id],
[Extent1].[Duration] AS [Duration],
[Extent1].[Difficulty] AS [Difficulty],
[Extent1].[Format] AS [Format]
FROM [dbo].[Questions] AS [Extent1]
WHERE ([Extent1].[Difficulty] = #p__linq__0)
AND ([Extent1].[Format] = #p__linq__1)
AND (2 = [Extent1].[Duration])) AS [Project1]
ORDER BY [Project1].[C1] ASC
UNION ALL
SELECT TOP (0) [Project3].[Id] AS [Id],
[Project3].[Duration] AS [Duration],
[Project3].[Difficulty] AS [Difficulty],
[Project3].[Format] AS [Format]
FROM (SELECT NEWID() AS [C1],
[Extent2].[Id] AS [Id],
[Extent2].[Duration] AS [Duration],
[Extent2].[Difficulty] AS [Difficulty],
[Extent2].[Format] AS [Format]
FROM [dbo].[Questions] AS [Extent2]
WHERE ([Extent2].[Difficulty] = #p__linq__2)
AND ([Extent2].[Format] = #p__linq__3)
AND (4 = [Extent2].[Duration])) AS [Project3]
ORDER BY [Project3].[C1] ASC) AS [UnionAll1]) AS [Distinct1]
UNION ALL
SELECT TOP (1) [Project7].[Id] AS [Id],
[Project7].[Duration] AS [Duration],
[Project7].[Difficulty] AS [Difficulty],
[Project7].[Format] AS [Format]
FROM (SELECT NEWID() AS [C1],
[Extent3].[Id] AS [Id],
[Extent3].[Duration] AS [Duration],
[Extent3].[Difficulty] AS [Difficulty],
[Extent3].[Format] AS [Format]
FROM [dbo].[Questions] AS [Extent3]
WHERE ([Extent3].[Difficulty] = #p__linq__4)
AND ([Extent3].[Format] = #p__linq__5)
AND (6 = [Extent3].[Duration])) AS [Project7]
ORDER BY [Project7].[C1] ASC) AS [UnionAll2]) AS [Distinct2]
UNION ALL
SELECT TOP (2) [Project11].[Id] AS [Id],
[Project11].[Duration] AS [Duration],
[Project11].[Difficulty] AS [Difficulty],
[Project11].[Format] AS [Format]
FROM (SELECT NEWID() AS [C1],
[Extent4].[Id] AS [Id],
[Extent4].[Duration] AS [Duration],
[Extent4].[Difficulty] AS [Difficulty],
[Extent4].[Format] AS [Format]
FROM [dbo].[Questions] AS [Extent4]
WHERE ([Extent4].[Difficulty] = #p__linq__6)
AND ([Extent4].[Format] = #p__linq__7)
AND (8 = [Extent4].[Duration])) AS [Project11]
ORDER BY [Project11].[C1] ASC) AS [UnionAll3]) AS [Distinct3];

Answer 2

Why not just get 4 (or max of the array) questions of each type and do the filtering on the client? With 16 results it should be cheap - you can use linq grouping to group them by minutes and then it should be easy. Another interesting point here is ordering... If you have more than 4 questions but you always order them when selecting would you ever see any other questions that just first 4?

Answer 3

My query is a little bit more complex. Basically, I ended up with the following:
context.Questions
.OrderBy(x => Guid.NewGuid())
.Where(x =>
x.AccessLevel >= accessLevel &&
x.Enabled == true &&
x.QuestionFormat == questionFormat
x.Theme.Id == themeId
)
.Select(x => new {
Answers = x.Answers.Select(y => new {
Correct = y.Correct,
Packs = y.Packs.SelectMany(z => z.Files, (z, v) => new {
Id = z.Id,
File = new { Key = v.Key, Mime = v.Mime }
}),
Text = y.Text
}),
Duration = x.Duration,
Note = x.Note,
Text = x.Text,
Packs = x.Packs.SelectMany(y => y.Files, (y, z) => new {
Id = y.Id,
File = new { Key = z.Key, Mime = z.Mime }
})
})
.GroupBy(x => x.Duration);
So I am ordering the questions randomly and filtering them.
Then I get, for each question, its answers ...
Each question and answer have some files associated with them.
I have all files in one table. This is why I have Packs and Files in my query.
I am only loading the Files Keys and Mimes. Not the data.
So you suggestion is to get all questions grouped by time, right?
In fact, after do the AccessLevel / Enabled / QuestionFormat / Theme filtering I will never have so many questions ...
So I can group them by Duration as I did in the end of this query.
What would be a fast way to take the number of questions of each duration according to the array I posted in the beginning?
Thank You,
Miguel