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Our model allows us to associate artists with comic books, but our current approach doesn't allow us to specify the role that the artist had. To do that, let's move from an "implicit" bridge table to an "explicit" one, by adding an entity class to our model for the bridge table.
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To follow along commiting your changes to this course, you'll need to fork the dotnet-comic-book-gallery-model repo. Then you can clone, commit, and push your changes to your fork like this:
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cd dotnet-comic-book-gallery-model
git checkout tags/v3.5 -b defining-many-to-many-relationship-with-an-explicit-bridge-entity
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Our model allows us to associate
artists with comic books, but
0:00
our current approach doesn't allow us to
specify the role that the artist had.
0:04
To do that, we need to move from an
implicit bridge table to an explicit one
0:09
by adding an entity class to our model for
the bridge table.
0:14
By defining an explicit bridge entity
class, we can include additional
0:18
properties beyond the properties that
are needed to define the relationship.
0:21
Bridge entities or
tables that include extra properties or
0:26
columns are sometimes
said to have payload.
0:30
Let's start with adding
the role entity class.
0:34
Right click on the Model's folder,
and select Add, Class.
0:36
Name the class Role and click Add.
0:42
Add the Public Access modifier
in the following properties, and
0:47
int property named ID.
0:52
Again this property will contain a unique
value for each role in our system.
0:55
A string property named, Name.
1:01
This is the name of the role.
1:06
Now let's add the bridge entity class.
1:09
I'll name the class ComicBookArtist.
1:15
At the public access modifier,
and the following properties,
1:24
an int property named Id.
1:29
This property will contain a unique value
for each comic book to artist relationship
1:32
in our system An int
property named ComicBookId.
1:37
This is a foreign key property to the
comic book that we're adding artists to.
1:44
An int property named ArtistId.
1:50
This is a foreign key
property to the artist
1:55
that we're adding to the comic book.
1:57
And an int property named RoleId.
2:00
This is a foreign key property to the role
that the artist had for this comic book.
2:05
Now let's add our navigation properties,
2:10
a ComicBook property named ComicBook.
2:14
And an ArtistProperty named Artist.
2:20
And lastly a Role property named Role.
2:26
Now we need to update the comic book and
2:34
artist entity classes to use
our new bridge entity class.
2:37
Starting with the comic book class,
2:44
change both references to the artist
class to ComicBookArtist.
2:47
And do the same for the artist class.
3:03
Now let's update our test data.
3:17
Just below our artists, let's add
3:24
a couple of roles, var role1 = new Role.
3:28
Name = "Script".
3:33
Var role2 = new Role.
3:38
Name = "Pencils".
3:43
To help with adding artist
to our comic books,
3:51
let's add a method to
the comic book entity class.
3:54
This will be a public method
that doesn't return a value.
4:04
So a return type of void.
4:07
And I'll name the method AddArtist.
4:10
It'll accept two parameters,
an Artist parameter of type artist
4:16
and a Role parameter of type role.
4:22
Now let's call the add method on the
artist navigation collection property and
4:26
pass in an instance of
the comic book artist class.
4:30
Artists.Add(new ComicBookArtist().
4:33
I'll set the Artists and Role properties
to their respective parameter values.
4:41
Artist = artist.
4:45
And Role = role.
4:49
Notice that we only had to explicitly set
the artist and role navigation properties.
4:53
Because we're adding the comic book
artist entity to a navigation property
4:58
on the comic book entity, EF is able
to set the comic book navigation
5:03
property value for us implicitly.With
that helper method in place,
5:07
we can easily replace our comicBook
Artists.Add method calls with Add artist,
5:12
passing in the appropriate role for
each artist.
5:17
AddArtist, and then.
5:24
Role1.
5:28
AddArtist.
5:34
And role2.
5:38
And again.
5:44
AddArtist.
5:47
role1.
5:51
AddArtist and role2, and
5:57
then the last comic book.
6:02
AddArtist.
6:07
Role1.
6:11
And lastly AddArtist.
6:13
Role2.
6:19
Now, we need to update our comicBooks
query to not only include our artist data,
6:25
but to include the associated artist and
role data for each bridge entity instance.
6:30
To do that, we can use the select operator
on the artist collection in order to
6:35
specify the path to the child property
that we want to include on the comic book
6:40
artist entity.
6:45
So, cb => cb.artist.select.
6:46
a => to, a.artist.
6:51
And a second call to the include method in
order to add the second child property,
6:58
the row property.
7:04
.Include(cb =>
7:05
cb.Artists.Select(a => a.Role.
7:09
Notice that we didn't need a third call to
the include method in order to populate
7:17
the artist's collection.
7:22
Every navigation property in the path
to the child navigation property that
7:23
we're populating will also be populated.
7:27
To complete our changes, let's update
the collection of artist name strings
7:31
to be a collection of artist role names.
7:36
We can do that by modifying the select
operator to return an interpolated string.
7:39
So, .Select (a
7:48
=> $"" quotes
7:53
{a.Artist.name}-
7:58
{a.Role.Name}.
8:05
Let's rename the artist names variable by
putting the cursor on the variable, and
8:14
pressing Ctrl+R, Ctrl+R.
8:19
Then change the name to
artistRoleNames and press Enter.
8:22
Let's also rename artistsDisplayText
to artistRolesDisplayText.
8:30
Let's test our app And
8:36
here's our three comic books along with
their respective artists and roles.
8:41
Let's review the database.
8:48
If you already have the tables folder open
be sure to refresh the list of tables.
8:53
The artist table remains unchanged, and
9:05
the roles table is what we'd
expect with ID and name columns.
9:09
The comic book artist table
contains four columns.
9:19
The ID primary key column and
three foreign key columns.
9:23
ComicBookId, ArtistId and RoleId.
9:27
Let's review the tables data.
9:32
We can see the ComicBookId 1 is
associated with ArtistId 1 and
9:38
2, and RoleId 1 and 2, respectively.
9:44
The same for ComicBookId 2.
9:48
And ComicBookId 3 is associated
with ArtistIds 1 and
9:54
3, and RoleIds 1 and 2 respectively.
9:59
Having the flexibility of an implicit or
explicit bridge entity when working with
10:05
many-to-many relationships, allows you
to start with the implicit approach and
10:10
switch to explicitly defining
the bridge entity when you need to.
10:14
After the break,
10:19
we'll see how we can use data annotations
to refine the generated database.
10:19
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