EF code first: using Linq with many-to-many relationship - asp.net-mvc

I am using MVC 4 with EF code first approach. I have two simple objects. These are their POCO classes:
public class Activity
{
//Primitive Properties
[HiddenInput]
public int Id { get; set; }
[Required]
public int LengthInMinutes { get; set; }
public string AdditionalInfo { get; set; }
[Required]
public bool Archive { get; set; }
//Navigation Properties
public virtual User User { get; set; }
public virtual ActivitySet ActivitySet { get; set; }
public virtual ICollection<Company> Companies { get; set; }
public virtual ICollection<Description> Descriptions { get; set; }
}
public class Company
{
//Primitive Properties
[HiddenInput]
public int Id { get; set; }
[Required]
public string Title { get; set; }
[Required]
public bool Archive { get; set; }
//Navigation Properties
public virtual ICollection<Activity> Activities { get; set; }
}
Now, I have generic list of activities which I iterate through using foreach loop. While looping I want to write a name for each Company related to the activity from the list. This is a code I came up with:
#foreach (Activity a in Model)
{
<p>#a.Companies.Where(d => d.Activities.FirstOrDefault(y => y.Id == a.Id)).Single()</p>
}
Unfortunately it gives me compilation error when I build the project. How can I then access details of the elements with many-to-many relationship

You could try it like so:
#foreach (Activity a in Model)
{
<p>#string.Join(",", a.Companies.Select(c => c.Title))</p>
}
It should give a list of all company titles of a given activity separated by a comma.

Related

MVC implementation of many to many relationships

This is done using MVC .net framework and entity framework "database first" approach. There is a many to many relationship between two tables. They are connected through third table that has combined key as id from first table and id from second table.
public class ManyToManyTable
{
[Required]
[Key, Column(Order=0)]
public int firsttableid { get; set; }
[Required]
[Key, Column(Order=1)]
public int secondtableid { get; set; }
public int something { get; set; }
[ForeignKey("firsttableid")]
public virtual FirstTable firstTable { get; set; }
[ForeignKey("secondtableid")]
public virtual SecondTable secondTable { get; set; }
}
First and Second table have some id which is primary key.
I want to create View and Controller method that enables master detail entry form for this ManyToManyTable. that would have FirstTable in Master and SecondTAble in details, and all to be saved in ManyToManyTable when button Save is pressed.
Of course, both First and Second Table have this property:
public virtual ICollection<ManyToManyTable> ManyToManyTables { get; set; }
What is the easiest way to implement cases like this one?
Thank you!
EF has a default conventions for many-to-many relationships. No need to create specific
mapping class. You have to include navigation properties in both "FirstTable" and "SecondTable" Class as shown below.
public class FirstTable
{
public FirstTable()
{
secondTableProperties = new HashSet<SecondTable>();
}
public int Id { get; set; }
public int MyProperty2 { get; set; }
public int MyProperty3 { get; set; }
public virtual ICollection<SecondTable> secondTableProperties { get; set; }
}
public class SecondTable
{
public SecondTable()
{
FirstTableProperties = new HashSet<FirstTable>();
}
public int Id { get; set; }
public int MyProperty2 { get; set; }
public int MyProperty3 { get; set; }
public virtual ICollection<FirstTable> FirstTableProperties { get; set; }
}
Remove mapping class from DBContext , only include above two classes. Build and run the application , EF will automatically create a Mapping table in SQL server. Usually the Mapping table contains only the primary keys of other two tables.
You can use Fluent API to take some control on the created mapping table
modelBuilder.Entity<FirstTable>()
.HasMany<SecondTable>(s => s.FirstTableProperties)
.WithMany(c => c.secondTableProperties)
.Map(cs =>
{
cs.MapLeftKey("FirstTableId");
cs.MapRightKey("SecondTableId");
cs.ToTable("ManyToManyTable");
});
If you want to work with a join table with additional properties, above mentioned many-to-many relationship won't work . In that case you will have to create two one-to-many relationships as shown below.
public class FirstTable
{
public int Id { get; set; }
public int MyProperty2 { get; set; }
public virtual ICollection<ManyToManyTable> manytomany { get; set; }
}
public class SecondTable
{
public int Id { get; set; }
public int MyProperty2 { get; set; }
public virtual ICollection<ManyToManyTable> manytomany { get; set; }
}
public ManyToManyTable
{
[Required]
[Key, Column(Order=0)]
public int firsttableid { get; set; }
[Required]
[Key, Column(Order=1)]
public int secondtableid { get; set; }
public int AdditionalProperty { get; set; }
public virtual FirstTable first { get; set; }
public virtual SecondTable Second { get; set; }
}

code first migration produces empty up down methods when add new model to existing database

I created models and performed code first migration which resulted in prepopulated up down methods.
However at a later stage I added new models to my application. The new models I added were Cart, OrderDetails and Order. I then typed add-migration for each of these models which as a result produced empty up down methods.
I would like to ask why are these up down methods empty when I added a new model.
I referenced these models in the dbcontext, the same dbcontext that referenced previously created models.
These are the new model classes that I added:
public class OrderDetail
{
public int OrderDetailId { get; set; }
public int OrderId { get; set; }
public int BookId { get; set; }
public int Quantity { get; set; }
public decimal UnitPrice { get; set; }
public virtual Book Books { get; set; }
public virtual Order Order { get; set; }
}
public class Cart
{
[Key]
public int RecordId { get; set; }
public string CartId { get; set; }
public int BookId{ get; set; }
public int Count { get; set; }
public virtual Book Book { get; set; }
}
class Order
{
public int OrderId { get; set; }
public string Username { get; set; }
public string FirstName { get; set; }
}
Here is my dbcontext
public BookStoreOnlineDB() : base("name=BookStoreOnlineDB")
{
}
public System.Data.Entity.DbSet<BookStoreOnline.Models.Book> Books { get; set; }
public System.Data.Entity.DbSet<BookStoreOnline.Models.Author> Authors { get; set; }
public System.Data.Entity.DbSet<BookStoreOnline.Models.BookStatus> BookStatus { get; set; }
public System.Data.Entity.DbSet<BookStoreOnline.Models.Genre> Genres { get; set; }
public System.Data.Entity.DbSet<BookStoreOnline.Models.Order> Orders { get; set; }
public System.Data.Entity.DbSet<BookStoreOnline.Models.OrderDetail> OrderDetails { get; set; }
public System.Data.Entity.DbSet<BookStoreOnline.Models.Cart>Carts { get; set; }
}
In summary, how do I populate the new up down methods with regards to the newly added Cart, OrderDetail and Detail models.
N.B. The orderDetails model and cart model reference the book model(book model contains had data migration performed on it at an earlier stage and contains populated up down methods).
New to this and would really appreciate help.
Thanks
Answer:
in PM Console:
add-migration initialcreate
//this included the newly added models e.g their ids,(cart,orderdetails, order models) to the up down methods

Entity framework migration of Collections

I have a model that looks like this
public abstract class Item
{
public int ItemId { get; set; }
public String Title { get; set; }
public String Description { get; set; }
public DateTime PurchaseDate { get; set; }
public ICollection<String> Pictures { get; set; }
public Int32 MinimumPrice { get; set; }
public DateTime Deadline { get; set; }
public Boolean VisibleBids { get; set; }
public Boolean Handled { get; set; }
public DateTime PlacementDate { get; set; }
public ApplicationUser User { get; set; }
}
In my controller I do
db.Items.ToList()
This leaves the Pictures field for all fetched objects null because its how the entity framework works. What is a good solution to fetch them in one query?
I hope you already done with Navigation properties between your tables, Now you just need to make your collection virtual and use the concept of eager loading when you need data from both the tables
public virtual ICollection<String> Pictures { get; set; }
and use include in linq like
db.Items.Include("Pictures").ToList()
So here by making virtual navigation you are saying that only take the data of related entity when you needed and whenever you need the data use the Include for eager loading.
For setting navigation properties please have a look on the code.
Suppose the scenario where we have a Post and on this we have multiple comments like
class Posts
{
public int PostsId { get; set; }
public string PostsDescription { get; set; }
public virtual ICollection<Comments> Comments { get; set; }
}
class Comments
{
public int CommentsId { get; set; }
public string CommentsDescription { get; set; }
public int PostsId { get; set; }
public virtual Posts Posts { get; set; }
}

Create object with many-to-many relationship in MVC 4

I am using MVC 4 with EF code first approach. I have two simple objects. These are their POCO classes:
public class Activity
{
//Primitive Properties
[HiddenInput]
public int Id { get; set; }
[Required]
public int LengthInMinutes { get; set; }
public string AdditionalInfo { get; set; }
[Required]
public bool Archive { get; set; }
//Navigation Properties
public virtual User User { get; set; }
public virtual ActivitySet ActivitySet { get; set; }
public virtual ICollection<Company> Companies { get; set; }
public virtual ICollection<Description> Descriptions { get; set; }
}
public class Company
{
//Primitive Properties
[HiddenInput]
public int Id { get; set; }
[Required]
public string Title { get; set; }
[Required]
public bool Archive { get; set; }
//Navigation Properties
public virtual ICollection<Activity> Activities { get; set; }
}
So there is a many-to-many relationship between Activity and Company entities. I am creating new Activity in my repository class, but when I assign a Company for the Activity like so:
activity.Companies.Add(company);
I get NullReference exception. I had a look around but according to this link:
BuildStarted.com
my approach seems to be right.
Why doesn't it work???
Before you can use Add() off Companies, it needs to be initialized.
activity.Companies = new List<Company>();
You could also initialize that in the contructor of Activity.
public class Activity
{
public Activity()
{
Companies = new List<Company>();
Descriptions = new List<Description>();
}
//Primitive Properties
[HiddenInput]
public int Id { get; set; }
[Required]
public int LengthInMinutes { get; set; }
public string AdditionalInfo { get; set; }
[Required]
public bool Archive { get; set; }
//Navigation Properties
public virtual User User { get; set; }
public virtual ActivitySet ActivitySet { get; set; }
public virtual ICollection<Company> Companies { get; set; }
public virtual ICollection<Description> Descriptions { get; set; }
}

MVC3 model set up - Code First EF

I'm trying to create a list of train journeys (among other things) in MVC, using code first Entity Framework and wondered how I could map foreign keys for the stations. The Journey model/table will have a DepartureStationID and an ArrivalStationID which will be foreign keys linking to one table/model, called Station.
Here is the code for both these models:
public class Station
{
public int StationID { get; set; }
public string StationName { get; set; }
public string StationLocation { get; set; }
}
public class Journey
{
public int JourneyID { get; set; }
public int DepartureID { get; set; }
public int ArrivalID { get; set; }
public int OperatorID { get; set; }
public string JourneyCode { get; set; }
public virtual Operator Operator { get; set; }
public virtual Station DepartureStation { get; set; }
public virtual Station ArrivalStation { get; set; }
}
There is another foreign key value in there, namely Operator and that has mapped successfully, but the departure and arrivals haven't, and return null values in the view: (#Html.DisplayFor(modelItem => item.DepartureStation.StationName).
When I looked in the database, there had been two additional fields created by EF:
DepartureStation_StationID
ArrivalStation_StationID
And the SQL relationship was between the station table and the two fields above, rather than DepartureID and ArrivalID
So, my question is - Do I need to do something different in the model when referencing the same table for two fields? I don't know why those additional fields were added so I presume I've set up the model incorrectly.
Thanks
For completeness, here's the same thing with fluent configuration.
public class MyDb : DbContext
{
public DbSet<Journey> Journeys { get; set; }
public DbSet<Operator> Operators { get; set; }
public DbSet<Station> Stations { get; set; }
protected override void OnModelCreating(DbModelBuilder builder)
{
builder.Entity<Journey>()
.HasRequired(j => j.DepartureStation)
.WithMany()
.HasForeignKey(j => j.DepartureID);
builder.Entity<Journey>()
.HasRequired(j => j.ArrivalStation)
.WithMany()
.HasForeignKey(j => j.ArrivalId);
// ... Same thing for operator ...
base.OnModelCreating(builder);
}
}
Edit: To address your above comment about the cascade delete, you can add .WillCascadeOnDelete(false) after .HasForeignKey() and that might help (although you'll then have to delete Journey records manually)
Add the folowing attributes on your navigation properties :
public class Journey
{
public int JourneyID { get; set; }
public int DepartureID { get; set; }
public int ArrivalID { get; set; }
public int OperatorID { get; set; }
public string JourneyCode { get; set; }
[ForeignKey("OperatorID")]
public virtual Operator Operator { get; set; }
[ForeignKey("DepartureID")]
public virtual Station DepartureStation { get; set; }
[ForeignKey("ArrivalID")]
public virtual Station ArrivalStation { get; set; }
}
And of course you need to regenerate your database in order to apply the new configuration.
Hope this will help.

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