I have data in two tables - an Albums table and a Tracks table. There is a one-to-many relationship between the two and the database structure mirrors my classes below. I'm trying to get an album and it's associated tracks based on the artist name (and I know that the data isn't normalized correctly - there should be an Artist entity, but my source data doesn't support that!)
public class Album
{
public string ArtistName { get; set;}
public string AlbumId { get; set; }
public string AlbumName { get; set; }
public int TrackCount { get; set; }
public string Year { get; set; }
public string Genre { get; set; }
public string CoverArt { get; set; }
public string Biography { get; set; }
// Navigation properties
public ICollection<Track> Tracks { get; set; }
}
public class Track
{
public string TrackId { get; set; }
public int TrackNumber { get; set; }
public string TrackName { get; set; }
public string AlbumId { get; set; }
}
My query to get a list of matching albums works fine:
var albums = DataContext.Albums.Where(a => a.ArtistName == artistName && a.AlbumName == album).ToList();
but the following statement doesn't work:
var tracks = DataContext.Tracks.Where(a => a.AlbumId == albums.Select(al => al.AlbumId).SingleOrDefault());
After it runs the tracks variable contains:
{System.Data.Entity.Infrastructure.DbQuery<MusicApp.Model.Track>} System.Linq.IQueryable<MusicApp.Model.Track> {System.Data.Entity.Infrastructure.DbQuery<MusicApp.Model.Track>}
I've tried appending .ToList() to my Linq statement and that just returns an error:
Additional information: Unable to create a constant value of type 'MusicApp.Model.Album'. Only primitive types or enumeration types are supported in this context.
I'm sure that the solution is really simple, but I'm new to Linq and just can't figure this one out. Any help would be much appreciated!
You can include the navigation properties you want:
var albumsWithTheirTracks = DataContext.Albums
.Where(a => a.ArtistName == artistName)
.Include("Tracks")
.ToList();
Doing this will populate each album's tracks collection with the related entities.
Your other option is to mark your Tracks navigation property as virtual in your Album model:
public virtual ICollection<Track> ...
Marking a nav property as virtual enables lazy loading and will bring along the related entities with your original query for album. If you don't choose to lazy load, you can use the Include method to specify which navigation properties you want to get along with the original album you're asking for.
var albums = DataContext.Albums.Where(a => a.ArtistName == artistName && a.AlbumName == album).ToList();
List<string> albumIds = albums.Select(s => s.AlbumId).Distinct().ToList();
var tracks = DataContext.Tracks.Where(w => albumIds.Contains(w.AlbumId)).ToList();
If you already have the album's data, why don't you try:
var tracks = from t in DataContext.Tracks
joins a in albums on a.AlbumId equals t.AlbumId
select t;
try this:
var query = from t in DataContext.Tracks
join a in DataContext.Albums on t.AlbumId equals a.AlbumId
where a.ArtistName == "artistName" && a.AlbumName == "album"
select t;
Unnecessary joins that everyone else are making, are a personal gripe of mine, as they make code hard to follow.
var query = from t in DataContext.Tracks
where t.Artists.Any(a => a.ArtistName == "artistName"
&& a.AlbumName == "album")
select t;
Related
I have already seen the answers to these questions In LINQ, how can I do an .OrderBy() on data that came from my .Include()?, ^ and ^, However, None is the answer to my question.
I have three entities: Letter, Person, LetterPerson as follows:
public class Letter
{
public int LetterId { get; set; }
public string Title { get; set; }
//MtoM
public ICollection<LetterPerson> LetterPersons { get; set; }
}
public class Person
{
public int PersonId { get; set; }
public string? FirstName { get; set; }
public string? LastName { get; set; }
//MtoM
public ICollection<LetterPerson> LetterPersons { get; set; }
}
public class LetterPerson
{
public int LetterPersonId { get; set; }
[ForeignKey("Letter")]
public int LetterId { get; set; }
[ForeignKey("Person")]
public int PersonId { get; set; }
public DateTimeOffset? AssignDate { get; set; }=DateTimeOffset.Now;
public Letter Letter { get; set; }
public Person Person { get; set; }
}
The Letter entity has Many To Many relationship with the Person entity by the LetterPerson entity. Now, I'd like to get a list of the person according to a specific letter's id and order by on the LetterPerson's id.
I have something like the following query in mind:
var PersonRec = await _dbContext.Persons
.Include(u => u.LetterPersons)
.Where(u => u.LetterPersons.Any(i => i.LetterId == LetterId))
.OrderBy(u => u.LetterPersons.LetterPersonId)
//.Include(u => u.LetterPersons.OrderBy(f=>f.LetterPersonId))
//.Where(u => u.LetterPersons.OrderBy(f=>f.LetterPersonId).Any(i => i.LetterId == LetterId))
//.OrderBy(u => u.LetterPersons.FirstOrDefault().LetterPersonId)
.ProjectTo<PersonDTO>(_mapperConfiguration).ToListAsync();
The above commented codes are the attempts that I made, but still the desired result was not achieved. I need .OrderBy(u => u.LetterPersons.LetterPersonId) , However, it clearly gives a compile error.
Question:
How should I correct the OrderBy part?
Just note that I have to send the query as a Data Transfer Object (PersonDTO) that the same as Person entity except FirstName field.
I use EF6 in .Net6.
If the combination (LetterId, PersonId) is unique in the joining table (typical for many-to-many), then the one-to-many relation from Person to LetterPerson for specific LetterId value becomes one-to-one, hence you can use Select or SelectMany with filter to get the single LetterPerson entry, which then could be used for ordering.
For isntance, using LINQ query syntax (more natural for such type of queries):
var query =
(
from p in _dbContext.Persons
from lp in p.LetterPersons
where lp.LetterId == LetterId
orderby lp.LetterPersonId
select p
)
.ProjectTo<PersonDTO>(_mapperConfiguration);
var result = await query.ToListAsync();
Note that you don't need Include in order to access related data inside LINQ to Entities query. Also for projection queries Includes are ignored.
I have following 2 classes. I am using EF code first with existing database.
public class Asset
{
[Key]
public decimal AssetId { get; set; }
[StringLength(20)]
public string AssetNumber { get; set; }
public decimal? DistrictId { get; set; }
public virtual District District { get; set; }
}
public class District
{
[Key]
public decimal? DistrictId { get; set; }
[StringLength(20)]
public string DistrictCode { get; set; }
[StringLength(255)]
public string Description { get; set; }
public virtual ICollection<Asset> Assets { get; set; }
}
I need to get the list of assets where DistrictId is not null.
I am using following c# code:
IQueryable<CustomClass> assets = dbContext.Assets
.Where(a => a.DistrictId != null)
.Select(a => new CustomClass
{
});
I believe using navigation properties, you don't need to explicitly say- Where(a => a.DistrictId != null). I am unsure about it. So can you suggest what is the better way of getting list of assets where DIstrictId is not null.
Thanks,
Since you are querying directly against Assets, you're not actually using the Asset <-> District relationship.
If you were trying to get a list of Assets with Districts included, or a list of Districts and their Assets, you would be using the Navigation properties.
For the result you're trying to get, as HaukurHaf said in his comment what you're doing is the most straightforward method.
That's as straight forward as can be, note that the DistrictID is only defining the relationship, it is not the navigation property, District is.
To verify you could do
// Eager loading all District properties and then filtering
List<CustomClass> assets = dbContext.Assets
.Include(a => a.District)
.ToList()
.Where(a => a.District != null)
.Select(a => new CustomClass {})
.ToList();
// Your initial query.
List<CustomClass> assets2 = dbContext.Assets
.Where(a => a.DistrictID != null)
.Select(a => new CustomClass {})
.ToList();
You will see they both eventually have the same list.
I have two objects: PhraseCategory and Phrase. Here's the classes:
public class PhraseCategory
{
public System.Guid PhraseCategoryId { get; set; } // PhraseCategoryId
public int PhraseCategoryShortId { get; set; } // PhraseCategoryShortId (Primary key)
public int PhraseCategoryGroupId { get; set; } // PhraseCategoryGroupId
public string Name { get; set; } // Name (length: 20)
// Reverse navigation
public virtual System.Collections.Generic.ICollection<Phrase> Phrases { get; set; } // Phrase.FK_PhrasePhraseCategory
}
public class Phrase : AuditableTable
{
public System.Guid PhraseId { get; set; } // PhraseId (Primary key)
public string English { get; set; } // English
public int? CategoryId { get; set; } // CategoryId
// Foreign keys
public virtual PhraseCategory PhraseCategory { get; set; } // FK_PhrasePhraseCategory
}
Can someone tell me how I could join these so that I am able to select all the phrases with for example a PhraseCategoryGroupId of 25.
Here's what I have right now but it does not take into account my need to also be able to select the Phrases with a PhraseCategory that has a PhraseCategoryGroupId:
List<Phrase> phrases;
var query = db.Phrases.AsQueryable();
if (options.CreatedBy != 0) query = query
.Where(w => w.CreatedBy == options.CreatedBy);
phrases = await query
.AsNoTracking()
.ToListAsync();
return Ok(phrases);
Note that I would like to get just a flat output (hope that makes sense). What I mean is a list that contains just:
PhraseId, English and CategoryId
This should get you what you need:
phrases = phrases.Where( x => x.PhraseCategory.PhraseCategoryGroupId == 25 )
.Select( x => new
{
PhraseId = x.PhraseId,
English = x.English,
CategoryId = x.CategoryId
});
Please note that you can also create instances of another type instead of the anonymous type which I am creating in the above query.
Also, the PhraseCategory will be lazy loaded in the above query since you have lazy loading enabled on the property: it is virtual. If you have lazy loading disabled globally, then you will need to use the Include method in your query. Then your query will become:
phrases = phrases.Include(x => x.PhraseCategory)
.Where( x => x.PhraseCategory.PhraseCategoryGroupId == 25 )
.Select( x => new
{
PhraseId = x.PhraseId,
English = x.English,
CategoryId = x.CategoryId
});
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
I have two objects that have been generated with an entity data model. The objects look like the following:
public class Song
{
public int ID { get; set; }
public string Title { get; set; }
public double Duration { get; set; }
}
public class AlbumSongLookup
{
public int ID { get; set; }
public int SongID { get; set; }
public int AlbumID { get; set; }
}
I need to get the Song objects for an Album using LINQ. I have the Album ID. Currently, I'm trying:
int albumID = GetAlbumID();
var results = from lookup in context.AlbumSongLookups
where lookup.AlbumID=albumID
select lookup;
I understand that I need to do a join. But what I'm not sure about is, how do I get the results to be Song objects with this LINQ query?
Thanks!
Does this query return what you're expecting?
var results = from lookup in context.AlbumSongLookups
join song in context.Songs on lookup.SongID equals song.ID
where lookup.AlbumID == albumID
select song;
I'm assuming the existence of context.Songs here.
does this not work?
from lookup in context.AlbumSongLookups
from songs in context.Song
where lookup.SongID == songs.ID && lookup.SongID == albumID
select songs
You could add a "Navigation Property" to your "Song" Entity, this way, you'll be able to directly access the corresponding Songs of a particular album. Entity framework will do the heavy lifting for you.
In your case, it returns a collection of lookups, what you need to do is look for the Song objects corresponding to them. this can be done using:
int albumID = GetAlbumID();
var results = from lookup in context.AlbumSongLookups
where lookup.AlbumID=albumID
select context.Songs.SingleOrDefault(s=>s.SongID==lookup.SongID);
Something like this should work:
int albumID = GetAlbumID();
var songIDs = from lookup in contenxt.AlbumSongLookups
where lookup.AlbumID == albumID
select lookup.SongID;
var results = from song in context.Songs
where song.SongID in SongIDs
select song;