Entity Framework - Join on many to many - join

I have a simple many to many relationship and I am wondering how you get data out of it. Here is the setup
Tables
Media
Media_Keyword (many to many map)
Keyword
Here is the code I have:
public List<Keyword> GetFromMedia(int mediaID)
{
var media = (from m in Connection.Data.Media
where m.id == mediaID
select m).First();
var keys = (from k in media.Media_Keyword
select new Keyword {ID = k.Keywords.id, Name = k.Keywords.keyword});
return keys.ToList();
}
Is there a way to do this better?

Usually, I select right from the many-to-many map.
var keys = from k in Connection.Data.Media_Keyword
where k.MediaID == mediaID
select k.Keywords;

I've not used the entity framework specifically, but can't you just combine them like this?
public List<Keyword> GetFromMedia(int mediaID)
{
return (from m in Connection.Data.Media
from k in m.Media_Keyword
where m.id == mediaID
select new Keyword {ID = k.Keywords.id, Name = k.Keywords.keyword}).ToList();
}

Response to Kleinux (Don't know why i can't add a comment to your question)
Sure you can, but it's not necessarly a good things, because context giving you a new "keyword". Then, if you try to update this or something thinking that you will update, context gonna see it as a new keyword and would create a new one instead of updating it.
** UPDATE
Sorry for my english, i'm french, well not french but from Quebec. I'm giving my 110%!!

Related

LINQ to Entities query with join inside method for use in MVC app

In my Person table is a RequestedLocation column which stores location IDs. The IDs match the LocationId column in the Locations table, the Locations table also has the text location names, in the LocatioName column.
In my view, I need to display the string LocationName in the view which has the Person model passed to it. The view will be displaying a List of people in a telerik grid. CUrrently it works great, except the RequestedLocation column is all integers.
I am populating all my grids with methods containing LINQ queries. Here is the method that currently works:
public List<Person> GetPeople()
{
var query = from p in _DB.Person.ToList()
select p;
return query.ToList();
}
Here is the regular SQL query that works, and I need to convert into LINQ:
SELECT ApplicantID
,FirstName
,LastName
,MiddleName
,DateofBirth
,Gender
,RequestedVolunteerRole
,RequestedVolunteerLocation
,l.LocationName
FROM Form.Person p
JOIN dbo.Location l ON p.RequestedVolunteerLocation = l.LocationID
Order BY ApplicantID
Here is my attempt to convert to LINQ:
public List<NewApplicantViewModel> GetPeople()
{
var query = from pl in _DB.Person.ToList()
join l in _Elig_DB.Locations.ToList() on pl.RequestedVolunteerLocation equals l.LocationID
select new
{
pl.RequestedVolunteerLocation = l.LocationName
};
return query.ToList();
The number of errors I get from this are numerous, but most are along the lines of:
Cannot convert from type Annonymous to Type List<NewAPplicantModel>
and
Invalid annonymous type declarator.
Please help, and thank you for reading my post.
Oh, and I have only been programming for a couple months, so if I am going about this all wrong, please let me know. Only thing I have to stick with is the table structure because it is an existing app that I am updating, and changing the location or person tables would have large consequences.
public List<NewApplicantViewModel> GetPeople()
{
var query = from pl in _DB.Person
join l in _Elig_DB.Locations on pl.RequestedVolunteerLocation
equals l.LocationID
select new NewApplicantViewModel
{
LocationName = l.LocationName,
otherPropery = p.Property
};
return query.ToList();
}
Beware of calling _DB.Person.ToList() it will load all persons from DB because ToList() immediately executes the query and the join would be performed in memory (not in DB).
The reason you are getting an error is you are projecting an anonymous type
select new
{
pl.RequestedVolunteerLocation = l.LocationName
};
Instead, you need to project a NewApplicantViewModel
select new NewApplicantViewModel
{
RequestedVolunteerLocation = l.LocationName
};

How do I use 2 include statements in a single MVC EF query?

I am trying to write a query that includes 2 joins.
1 StoryTemplate can have multiple Stories
1 Story can have multiple StoryDrafts
I am starting the query on the StoryDrafts object because that is where it's linked to the UserId.
I don't have a reference from the StoryDrafts object directly to the StoryTemplates object. How would I build this query properly?
public JsonResult Index(int userId)
{
return Json(
db.StoryDrafts
.Include("Story")
.Include("StoryTemplate")
.Where(d => d.UserId == userId)
,JsonRequestBehavior.AllowGet);
}
Thank you for any help.
Try to flatten your hierarchy if it works for you. Here is a sample, and you may want to customize it for your needs.
var result = from c in db.Customers
join o in db.Orders
on c equals o.Customers
select new
{
custid = c.CustomerID,
cname = c.CompanyName,
address = c.Address,
orderid = o.OrderID,
freight = o.Freight,
orderdate = o.OrderDate
};
If flattering does not meet your requirements then you need to use query that returns a Nested Group. Finally, look at the following link for more references - LINQ Query Expressions .

How to specify a condition in an Entity Framework join?

I have a Blogs table related to BlogComments table with a FK.
I need to get, through Linq, all the BlogComments items that match a certain flag
If i do:
db.Blogs.Where(b => b.BlogComments.Where(bc=>bc.Where(bc.Flag1==true));
I get "Cannot implicity convert type IEnumerable to bool"
Which is the best way to solve this problem?
Because this expression:
b.BlogComments.Where(...)
returns an IEnumerable (of BlogComments), but you are then passing it into this method:
db.Blogs.Where(...)
which expects a function that returns a bool, not an IEnumerable.
You probably need something like this:
var blogId = 5;
db.BlogComments.Where(bc => bc.BlogId == blogId && bc.Flag1 == true)
If you need to select comments from multiple blogs, then you could try using Contains:
var blogIds = new [] {1,2,3,4,5};
db.BlogComments.Where(bc => blogIds.Contains(bc.BlogId) && bc.Flag1 == true)
If you want to place criteria on the set of blogs, as well as the comments, then you could do this in one query using a join:
var query = from b in db.Blogs
join c in db.BlogComments on c.Blog equals b
where b.SomeField == "some value"
&& c.Flag1 == true
select c;
You could write it in LINQ form.
var blogs = from b in db.Blogs
join c in db.BlogComments
on b.BlogId equals c.BlogId
where c.Flag1
select b;
If you have a composite key you can write
on new { A = b.BlogKey1, B = b.BlogKey2 }
equals new { A = c.CommentKey1, B = c.CommentKey2 }
If it were me, I would just have another DbSet in your DbContext.
DbSet<BlogComment> BlogComments
and just search through there without going through Blogs.
db.BlogComments.Where(bc => bc.Flag1 == true);
If anyone knows if there's anything wrong in doing so, then I'm all ears :)

How to join multiple tables using LINQ-to-SQL?

I'm quite new to linq, so please bear with me.
I'm working on a asp.net webpage and I want to add a "search function" (textbox where user inputs name or surname or both or just parts of it and gets back all related information). I have two tables ("Person" and "Application") and I want to display some columns from Person (name and surname) and some from Application (score, position,...). I know how I could do it using sql, but I want to learn more about linq and thus I want to do it using linq.
For now I got two main ideas:
1.)
var person = dataContext.GetTable<Person>();
var application = dataContext.GetTable<Application>();
var p1 = from p in Person
where(p.Name.Contains(tokens[0]) || p.Surname.Contains(tokens[1]))
select new {Id = p.Id, Name = p.Name, Surname = p.Surname}; //or maybe without this line
//I don't know how to do the following properly
var result = from a in Application
where a.FK_Application.Equals(index) //just to get the "right" type of application
//this is not right, but I don't know how to do it better
join p1
on p1.Id == a.FK_Person
2.) The other idea is just to go through "Application" and instead of "join p1 ..." to use
var result = from a in Application
where a.FK_Application.Equals(index) //just to get the "right" type of application
join p from Person
on p.Id == a.FK_Person
where p.Name.Contains(tokens[0]) || p.Surname.Contains(tokens[1])
I think that first idea is better for queries without the first "where" condition, which I also intended to use. Regardless of what is better (faster), I still don't know how to do it using linq. Also in the end I wanted to display / select just some parts (columns) of the result (joined tables + filtering conditions).
I really want to know how to do such things using linq as I'll be dealing also with some similar problems with local data, where I can use only linq.
Could somebody please explain me how to do it, I spent days trying to figure it out and searching on the Internet for answers.
var result = from a in dataContext.Applications
join p in dataContext.Persons
on p.Id equals a.FK_Person
where (p.Name.Contains("blah") || p.Surname.Contains("foo")) && a.FK_Application == index
select new { Id = p.Id, Name = p.Name, Surname = p.Surname, a.Score, a.Position };
Well as Odrahn pointed out, this will give you flat results, with possibly many rows for a single person, since a person could join on multiple applications that all have the same FK. Here's a way to search all the right people, and then add on the relevant application to the results:
var p1 = from p in dataContext.Persons
where(p.Name.Contains(tokens[0]) || p.Surname.Contains(tokens[1]))
select new {
Id = p.Id, Name = p.Name, Surname = p.Surname,
BestApplication = dataContext.Applications.FirstOrDefault(a => a.FK_Application == index /* && ???? */);
};
Sorry - it looks like this second query will result in a roundtrip per person, so it clearly won't be scalable. I assumed L2S would handle it better.
In order to answer this properly, I need to know if Application and Person are directly related (i.e. does Person have many Applications)? From reading your post, I'm assuming that they are because Application seems to have a foreign key to person.
If so, then you could create a custom PersonModel which will be populated by the fields you need from the different entities like this:
class PersonModel
{
string Name { get; set; }
string Surname { get; set; }
List<int> Scores { get; set; }
List<int> Positions { get; set; }
}
Then to populate it, you'd do the following:
// Select the correct person based on Name and Surname inputs
var person = dataContext.Persons.Where(p => p.Name.Contains("firstname") || p.Name.Contains("surname")).FirstOrDefault();
// Get the first person we find (note, there may be many - do you need to account for this?)
if (person != null)
{
var scores = new List<int>();
var positions = new List<int>();
scores.AddRange(person.Applications.Select(i => i.Score);
positions.AddRange(person.Applications.Select(i => i.Position);
var personModel = new PersonModel
{
Name = person.Name,
Surname = person.Surname,
Scores = scores,
Positions = positions
};
}
Because of your relationship between Person and Application, where a person can have many applications, I've had to account for the possibility of there being many scores and positions (hence the List).
Also note that I've used lambda expressions instead of plain linqToSql for simple selecting so that you can visualise easily what's going on.

How to create foreign key relationships with the Entity Framework?

I want to create a new row in my database on a table that has a couple of foreign key relationships and I haven't been able to get a handle on what order and what calls need to be made. This is what I have so far:
db.Models.Order order = DB.Models.Order.CreateOrder( apple );
order.CustomerReference.Attach( ( from c in db.Customer where c.Id == custId select c ).First() );
db.SaveChanges();
The code is failing on the second line there, saying:
Attach is not a valid operation when
the source object associated with this
related end is in an added, deleted,
or detached state. Objects loaded
using the NoTracking merge option are
always detached.
Any ideas?
(Thanks John for the grammar fixes)
So I figured it out. This is what you have to do:
db.Models.Order order = DB.Models.Order.CreateOrder( apple );
order.Customer = (from c in db.Customer where c.Id == custId select c).First();
db.SaveChanges();
I hope that helps people.
Why not use entity references? Your method will cause an extra SELECT statement.
A much nicer way is to use the CustomerReference class and an EntityKey.
order.CustomerReference = new System.Data.Objects.DataClasses.EntityReference<Customers>();
order.CustomerReference.EntityKey = new EntityKey("ModelsEntities.Customers", "Id", custId);
For update here is some sample code:
using (var ctx = new DataModelEntities())
{
var result = (from p in ctx.UserRole.Where(o => o.UserRoleId == userRole.UserRoleId)
select p).First();
result.RolesReference.EntityKey = new EntityKey("DataModelEntities.Roles",
"RoleId", userRole.RoleId);
result.UserRoleDescription = userRole.UserRoleDescription;
ctx.SaveChanges();
}

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