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Building objects with many-to-many relationship using Dapper

Consider an Sqlite database, whose partial schema is shown below (we are not considering the Book_Tag table here). Note the many-to-many relationship between media items and tags using the link table Media_Tag:
An object model for these tables is as follows:
public enum MediaType
{
Dvd,
BluRay,
Cd,
Vhs,
Vinyl,
Other
}
public class MediaItem
{
public MediaType type { get; set; }
public long number { get; set; }
public int runningTime { get; set; }
public int releaseYear { get; set; }
public ICollection<Tag> tags { get; set; }
}
public class Tag
{
public string name { get; set; }
}
currently, Dapper is being used to read from the Media table, but without considering tags. The code is as follows:
public IEnumerable<MediaItem> readAll()
{
using (var db = new SqliteConnection(this.connectionString))
{
db.Open();
var sql = "SELECT * FROM Media;";
return db.Query<MediaItem>(sql);
}
}
public MediaItem readById(int id)
{
using (var db = new SqliteConnection(this.connectionString))
{
db.Open();
var sql = "SELECT * FROM Media WHERE id = #id;";
var #params = new { id = id };
return db.Query<MediaItem>(sql, #params).First();
}
}
How to change this so that the tag property of MediaItem is considered when creating the objects, for both cases (read by id and read all rows from the table)? Is a join query required? I'm sure Dapper has a way of doing this nicely, but I don't know how it's done.

You are not interested in anything from the link table so something like this SQL should do:
SELECT M.Id, M.title, M.type, M.Number, M.image, M.runningTime, M.releaseYear, T.Id, T.Name FROM Media as M
INNER JOIN Media_Tag AS MT ON M.id = MT.mediaId
INNER JOIN Tags AS T ON T.id = MT.tagId
If SqLite allows you can use M.*, T.* instead.
I have taken the liberty to add Id properties to your entity classes. I think you are going to need it, otherwise all your tags will be different instead of being unique. You might make it work without it, but it should make your life easier.
public class MediaItem
{
public int Id { get; set; } // New
public MediaType type { get; set; }
public long number { get; set; }
public int runningTime { get; set; }
public int releaseYear { get; set; }
public ICollection<Tag> tags { get; set; }
}
public class Tag
{
public int Id { get; set; } // New
public string name { get; set; }
}
Since both your entity classes have a unique id, you will have to pick them up and make sure they are unique going through the results. We do that by using dictionaries to keep them. I'm only showing the ReadAll, you should be able to do ReadById accordingly.
string sql = "<see above>";
using (var db = new SqliteConnection(this.connectionString))
{
var mediaDictionary = new Dictionary<int, Media>();
var tagDictionary = new Dictionary<int, Tag>();
var list = db.Query<Media, Tag, Media>(
sql,
(media, tag) =>
{
Media mediaEntry;
if (!mediaDictionary.TryGetValue(media.Id, out mediaEntry))
{
// Haven't seen that one before, let's add it to the dictionary
mediaEntry = media;
mediaDictionary.Add(mediaEntry.Id, mediaEntry);
}
Tag tagEntry;
if (!tagDictionary.TryGetValue(tag.Id, out tagEntry))
{
// Haven't seen that one before, let's add it to the dictionary
tagEntry = tag;
tagDictionary.Add(tagEntry.Id, tagEntry);
}
// Add the tag to the collection
mediaEntry.Tags.Add(tagEntry);
return mediaEntry;
},
splitOn: "Id") // This default and could be omitted
.Distinct()
.ToList();

writing LINQ query to pivot the result

I have developed a LINQ query. Now my requirement is to create pivot query from it. I am new to LINQ, I do not know how to proceed further. Please see the attached the attached result image.
public JsonResult SchoolNikashaRpt()
{
try
{
var temp = (from n in db.Nikashas
join s in db.Schools on n.SchoolId equals s.SchoolId
join k in db.Programs on n.ProgramId equals k.ProgramId
orderby n.SchoolId
select new RptSchoolsNikashaViewModel
{
SCHOOL_NAME = s.SCHOOL_NAME
,PROGRAM_NAME = k.PROGRAM_NAME
,MAPPED_AMOUNT = n.MAPPED_AMOUNT
}).ToList();
return Json(temp, JsonRequestBehavior.AllowGet);
}
catch (Exception)
{
throw;
}
}
Involved model classes are as follows::
public class NikashaModels
{
[Key]
[DatabaseGenerated(DatabaseGeneratedOption.Identity)]
public int NIKASHAId { get; set; }
[Required]
public decimal MAPPED_AMOUNT { get; set; }
[ForeignKey("ProgramId")]
public ProgramModels Program { get; set; }
public int ProgramId { get; set; }
[ForeignKey("SchoolId")]
public SchoolModels School { get; set; }
public int SchoolId { get; set; }
}
public class SchoolModels
{
[Key]
[DatabaseGenerated(DatabaseGeneratedOption.Identity)]
public int SchoolId { get; set; }
public string SCHOOL_NAME { get; set; }
}
public class ProgramModels
{
[Key]
public int ProgramId { get; set; }
public string PROGRAM_NAME { get; set; }
}

So you have Nikashas, Schools and Programs. There is a one-to-many relation: Every Nikasha has exactly one School, and exactly one Program, namely the School and Program that the foreign key refers to. In the other direction: Every School has zero or more Nikashas, every Program has zero or more Nikashas.
You want for every Nikasha, some properties, and some information of its School and its Program. For this you can use one of the overloads of Enumerable.Join, if more than two tables are involved it is easier to do this using Enumerab.Select
You didn't mention it, but because I see the word db, it seems to me that you are fetching the data from a DbContext, so your Enumerables are IQueryable<...>. This doesn't influence the answer very much:
IQueryable<Nikasha> nikashas = db.Nikashas
// only if you don't want all Nikashas:
.Where(nikasha => ...);
IQueryable<School> schools = ...
IQueryable<Program> programs = ...
var result = nikashas.Select(nikasha => new RptSchoolsNikashaViewModel
{
// Get the name of the one and only School of this nikasha
SchoolName = schools
// Keep only the School that the foreign key refers to:
.Where(school => school.Id == nikasha.SchoolId)
// Select the name of the School
.Select(school => school.SchoolName)
// and take the first element
.FirstOrDefault(),
// Do something similar with the program name:
ProgramName = programs.Where(program => program.Id == nikasha.ProgramId)
.Select(program => program.ProgramName)
.FirstOrDefault(),
MappedAmount = nikasha.MappedAmount,
});
In words: from every Nikasha, make one new RptSchoolsNikashaViewModel. For this RptSchoolsNikashaViewModel use the MappedAmount of the Nikasha. To get the SchoolName, take all Schools that have a primary key value equal to the foreign key in the Nikasha. From the remaining schools (probably only one), take the name of the School. Finally take the first item from the remaining school names. Do something similar for ProgramNames.

How to hide items, in an API reply, from a db query?

I'm currently using MVC with EF to have a small server with API querying a SQL database. But in the API reply I'm not able to hide some parameters.
The main object
public class AssetItem
{
[Key]
public Int32 AssetId { get; set; }
public String AssetName { get; set; }
public int OdForeignKey { get; set; }
[ForeignKey("OdForeignKey")]
public OperationalDataItem OperationalDataItem { get; set; }
}
The other one:
public class OperationalDataItem
{
[Key]
public Int32 OperationalDataId { get; set; }
public String Comunity { get; set; }
public List<AssetItem> AssetItems { get; set; }
}
From what I have read, this should be ok, I have also set the context:
public AssetContext(DbContextOptions<AssetContext> options) : base(options)
{}
public DbSet<AssetItem> AssetItems { get; set; }
public DbSet<OperationalDataItem> OperationalDataItems { get; set; }
protected override void OnModelCreating(ModelBuilder modelBuilder)
{
modelBuilder.Entity<AssetItem>().HasOne(p =>
p.OperationalDataItem).WithMany(b => b.AssetItems).HasForeignKey(p =>
p.OdForeignKey);
}
And the seeding in program.cs
context.AssetItems.Add(
new AssetItem { AssetName = "Test test", OdForeignKey = 1,
OperationalDataItem =
new OperationalDataItem {Comunity = "Comunity1" }});
So calling the API this results in:
{ "assetId":3,
"assetName":"Test test",
"odForeignKey":1,
"operationalDataItem":null }
From what I read this is because of the lazy loading, how can I hide the result operationalDataItem?
In case is not possible i have of course try to query for it and give it back and it give something like:
{ "assetId":3,
"assetName":"Test test",
"odForeignKey":1,
"operationalDataItem":
{ "operationalDataId":1,
"comunity":"Comunity1",
"assetItems":[
But in this case I would like to hide "assetsItems" in the reply to the FE.
How can I hide those parameters?
The API is quite simple, just an example code:
var todoItem = await _context.AssetItems.FindAsync((Int32)id);
var item = _context.OperationalDataItems.Find((Int32)todoItem.OdForeignKey);
todoItem.OperationalDataItem = item;
return todoItem

If you want to fetch data from the database, but you only want to fetch some properties, use Select. Usually this is more efficient than using Find, because you'll only transfer the data that you actually plan to use.
To fetch some properties of the assetItem that has primary key assetItemId:
var result = dbContext.AssetItems
.Where(assetItem => assetItem.AssetItmId = assetItemId)
.Select(assetItem => new
{
// Select only the properties that you plan to use
Id = assetItem.AssertItemId,
Name = assetItem.Name,
OperationalData = new
{
// again, select only the properties that you plan to use
Id = assetItem.OperationalData.OperationalDataId,
Community = assetItem.OperationalData.Community,
},
})
.FirstOrDefault();
Or the other way round:
Fetch several properties of all (or some) OperationalDataItems, each with some properties of all (or some) of its AssetItems:
var result = dbContext.OperqationalDataItems
.Where(operationalDataItem => ...) // only if you don't want all
.Select(operationalDataItem => new
{
Id = operationalDataItem.Id,
Community = operationalDataItem.Community
AssetItems = operationalDataItem.AssetItems
.Where(assetItem => ...) // only if you don't want all its assetItems
.Select(assetItem => new
{
// Select only the properties you plan to use:
Id = assetItem.Id,
...
// not useful: you know the value of the foreign key:
// OperationalDataId = assetItem.OperationalDataId,
})
.ToList();
})
.ToList(); // or: FirstOrDefault if you expect only one element
Entity framework knows your one-to-many relation and is smart enough to know which (group-)join is needed for your query.
Some side remarks
You've declare your many-relation a List<AssetItem>. Are you sure that operationalDataItem.AssetItems[4] has a defined meaning? Wouldn't it be better to stick to the entity framework code first conventions? This would also eliminate the need for most attributes and / or fluent API
public class OperationalDataItem
{
public int Id { get; set; }
public String Comunity { get; set; }
...
// Every OperationalDataItem has zero or more AssetItems (one-to-many)
public virtual ICollection<AssetItem> AssetItems { get; set; }
}
public class AssetItem
{
public int Id { get; set; }
public String Name { get; set; }
...
// every AssetItem belongs to exactly one OperationalDataItem, using foreign key
public int OperationDataItemId { get; set; }
public virtual OperationalDataItem OperationalDataItem { get; set; }
}
In entity framework the columns of a table are represented by the non-virtual properties. The virtual properties represent the relations between the tables (one-to-many, many-to-many)
Because I stuck to the conventions, no attributes nor fluent API is needed. Entity framework is able to detect the one-to-many relation and the primary and foreign keys. Only if I am not satisfied with the names or the types of the columns I would need fluent API.

How can I get the count of a list in an Entity Framework model without including/loading the entire collection?

I have a model in Entity Framework Core that goes something like this:
public class Anime
{
public int EpisodeCount { get { return Episodes.Count() } }
public virtual ICollection<Episode> Episodes { get; set; }
}
I'm having the issue of EpisodeCount being 0. The solution currently is to run a .Include(x => x.Episodes) within my EF query, but that loads the entire collection of episodes where it's not needed. This also increases my HTTP request time, from 100ms to 700ms which is just not good.
I'm not willing to sacrifice time for simple details, so is there a solution where I can have EF only query the COUNT of the episodes, without loading the entire collection in?
I was suggested to do this
var animeList = context.Anime.ToPagedList(1, 20);
animeList.ForEach(x => x.EpisodeCount = x.Episodes.Count());
return Json(animeList);
but this also returns 0 in EpisodeCount, so it's not a feasible solution.

You need to project the desired data into a special class (a.k.a. ViewModel, DTO etc.). Unfortunately (or not?), in order to avoid N + 1 queries the projection must not only include the count, but all other fields as well.
For instance:
Model:
public class Anime
{
public int Id { get; set; }
public string Name { get; set; }
// other properties...
public virtual ICollection<Episode> Episodes { get; set; }
}
ViewModel / DTO:
public class AnimeInfo
{
public int Id { get; set; }
public string Name { get; set; }
// other properties...
public int EpisodeCount { get; set; }
}
Then the following code:
var animeList = db.Anime.Select(a => new AnimeInfo
{
Id = a.Id,
Name = a.Name,
EpisodeCount = a.Episodes.Count()
})
.ToList();
produces the following single SQL query:
SELECT [a].[Id], [a].[Name], (
SELECT COUNT(*)
FROM [Episode] AS [e]
WHERE [a].[Id] = [e].[AnimeId]
) AS [EpisodeCount]
FROM [Anime] AS [a]

LINQ to Entities query error

I am encountered an error that I am not familier with. I tried to google with no success.
I wrote the following query where I am having this error.
The entity or complex type 'MyWebProject.Models.UserDetail' cannot be constructed in a LINQ to Entities query.
The query:
UsersContext db = new UsersContext();
var userdata = (from k in db.UserDetails
where k.UserId == WebSecurity.CurrentUserId
select new UserDetail()
{
FullName = k.FullName,
Email = k.Email,
About = k.About,
Link = k.Link,
UserSchool = new School()
{
SchoolId = k.UserSchool.SchoolId,
SchoolName = k.UserSchool.SchoolName
},
UserCourse = new Course()
{
CourseId=k.UserCourse.CourseId,
CourseName=k.UserCourse.CourseName
},
Country=k.Country
}).FirstOrDefault();
Class:
public class UserDetail
{
public int Id { get; set; }
public int UserId { get; set; }
public string FullName { get; set; }
public string Link { get; set; }
public bool? Verified { get; set; }
public string Email { get; set; }
public string About { get; set; }
public School UserSchool { get; set; }
public Course UserCourse { get; set; }
public string Country { get; set; }
}
public class School
{
public int SchoolId { get; set; }
public string SchoolName { get; set; }
public string Country { get; set; }
}
public class Course
{
public int CourseId { get; set; }
public string CourseName { get; set; }
public School School { get; set; }
}
Any idea what went wrong??

It looks like it is due to how you are creating the complex properties School and Course in the middle of the query. It would be better to select the User (remove the select transformation), then use navigation properties to access those objects instead of building them manually. The navigation are meant for this as long as you have the proper relations built with foreign keys.
UsersContext db = new UsersContext();
var userdata = (from k in db.UserDetails
where k.UserId == WebSecurity.CurrentUserId})
.FirstOrDefault();
// access navigation properties which will perform the joins on your behalf
// this also provides for lazy loading which would make it more effecient. (it wont load the school object until you need to access it)
userdata.School
userdata.Course
MSDN article about navigation properties: http://msdn.microsoft.com/en-us/library/vstudio/bb738520(v=vs.100).aspx

This should give you what you want. It will load your objects as part of the query (and not rely on lazy loading).
UsersContext db = new UsersContext();
var userdata = db.UserDetails.Include(x => x.UserSchool)
.Include(x => x.UserCourse)
.Include(x => x.Country)
.Where(x => x.UserId == WebSecurity.CurrentUserId)
.FirstOrDefault();

I think it's because your entity has the same name of the object you're trying to create. Try renaming the object you want to return back. If you want to return the same type as your entity try the eager loading with .Include("relationshipname") feature.

A great answer from #Yakimych is given below.
You cannot (and should not be able to) project onto a mapped entity. You can, however, project onto an annonymous type or onto a DTO:
public class ProductDTO
{
public string Name { get; set; }
// Other field you may need from the Product entity
}
And your method will return a List of DTO's.
public List<ProductDTO> GetProducts(int categoryID)
{
return (from p in db.Products
where p.CategoryID == categoryID
select new ProductDTO { Name = p.Name }).ToList();
}
Mapped entities in EF basically represent database tables. If you project onto a mapped entity, what you basically do is partially load an entity, which is not a valid state. EF won't have any clue how to e.g. handle an update of such an entity in the future (the default behaviour would be probably overwriting the non-loaded fields with nulls or whatever you'll have in your object). This would be a dangerous operation, since you would risk losing some of your data in the DB, therefore it is not allowed to partially load entities (or project onto mapped entities) in EF.
For more details please go to the following link:
The entity cannot be constructed in a LINQ to Entities query