Creating Entities

9:22 with James Churchill

Let's review how to use EF to create entities and implement the console application's Repository `AddComicBook` method.

Teacher's Notes
Video Transcript
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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:

git clone <your-fork>
cd dotnet-comic-book-gallery-model
git checkout tags/v5.2 -b creating-entities

Exercise Solution

Here's the solution to the exercise that's suggested at the end of the video.

foreach (ComicBookArtist artist in comicBook.Artists)
{
    if (artist.Artist != null && artist.Artist.Id > 0)
    {
        context.Entry(artist.Artist).State = EntityState.Unchanged;
    }

    if (artist.Role != null && artist.Role.Id > 0)
    {
        context.Entry(artist.Role).State = EntityState.Unchanged;
    }
}

0:00
As we've seen in an earlier video, adding an entity to the database
0:04
is as simple as adding it to it's respective dbset property.
0:07
And, calling the save changes method.
0:10
We can use that knowledge to implement the add comic book method.
0:14
If you're following along, see the teacher's notes for
0:17
a link to download the project files.
0:20
The add comic book method is in the repository class which is located in
0:24
the data folder.
0:35
To start, we need to get an instance of the context class by calling the get
0:39
context method within a using statement.
0:41
Using (Context context = GetContext()).
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Let's pick the definition for
0:50
the GetContext method by placing the cursor
0:55
on the method call and pressing alt F12.
1:03
The GetContext method instantiates an instance of the context class and
1:08
sets its database log property to an anonymous method
1:12
that writes messages to the output window, using the Debug.WriteLine method.
1:19
Press the Escape key to close the peak definition window.
1:24
Now let's finish implementing the add comic book method.
1:27
First, call the context comic books DB sets add method,
1:31
passing in the comic book parameter.
1:39
Then call the context save changes method.
1:44
Let's test our changes by setting a break point just inside of the add comic
1:49
book method, and run in the app.
1:58
Enter A to add a comic book, then Enter one to select the Bones series four,
2:04
for the issue number, skip the description for now,
2:09
1/1/2016 for the published on date.
2:13
Skip the average rating, three for the artist and two for the role.
2:23
Here we are at our break point in In the Locals window notice that while the series
2:27
ID Foreign Key property value is three, the series navigation property is null.
2:33
As we discussed in an earlier video this is one of the advantages
2:37
of defining Foreign Key properties in our entities.
2:40
We can associate a related entity by just setting their foreign key property.
2:45
In fact, when adding entities, this is exactly the approach that we want to use.
2:50
If the series navigation property wasn't null when adding the comic book entity to
2:54
the context, we'd inadvertently add a duplicate series record to the database.
3:00
Let's look at an example.
3:02
We're still at our breakpoint.
3:04
I'll press F5 to complete the process of adding the comic book.
3:07
And enter q to quit the app.
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I'll set the series navigation property to an instance of the series' entity.
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Comicbook series equals new series
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with an ID of three and a title of bone.
3:34
Now run the app.
3:39
And enter a to add another comicbook.
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Enter 1 to select the bones series, 5 for the issue number,
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2/1/2016 for the published on date,
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3 for the artist in 2 for the row.
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We'll add our break point again.
3:59
Press F5 to continue execution.
4:03
Enter A again to start adding another comic book.
4:07
Notice that the Bone series is now listed twice.
4:12
Go ahead and switch back to Visual Studio and stop the app.
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If we open the sequel server object Explorer window,
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expand the ComicBookLibraryManager database, and
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tables and view the data for the series table,
4:34
we can see that we definitely have two rows now.
4:38
Why is this happening?
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One of the features of the context is that it tracks the state of and
4:44
the changes to each entity that it has currently loaded.
4:48
This is called change tracking.
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When we add an entity to the context using the DbSet.Add Method,
4:54
the entity is added to the context and its state is set to added.
5:01
We can verify that by retrieving the entity's entry in the context
5:04
using the entry method and checking the value with
5:07
the state property just before calling the Save Changes method.
5:16
Var comicBookEntry = context.Entry(comicBook).
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Let's also retrieve the entry for the series entity.
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Var seriesEntry =
5:32
context.Entry(comicBook.Series).
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Remove the previous break point Set a new break point on the line of code
5:45
that's calling the Save Changes method and run the app.
5:55
Enter A to add another comic book, 1 to select the Bone series,
6:02
6 for the issue number 3/1/2016 for
6:08
the published on date, 3 for the artist, and 2 for
6:13
the role, In the Locals window,
6:18
we can see that the comic book entry state property is set to added.
6:22
That's the expected behavior.
6:25
What might not be expected,
6:26
is that the series entries state property is also set to added.
6:34
When adding an entity to the context by calling the D.B.
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said add method, E.F. what not only set to entity state to added but
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also each of the entities related or child entities.
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When the Save Changes method is called, each of the entities in the context that
6:50
has a state of added will be inserted into the database.
6:54
That's true even for the Bone series entity that has an ID property value
6:59
which in this case is three that matches an existing record in the database.
7:04
The Id property value is simply ignored.
7:10
Let's fix this problem.
7:16
After calling the Add method let's check if the series property isn't null and
7:21
the series entity Id property value is greater than zero.
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If comic.Book.Series does not equal null and
7:29
comicBook.Series.Id is greater than zero.
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An Id property value greater than zero
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indicates that the entity already exists in the database.
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Then let's retrieve the entry for
7:44
the series entity instead state property to unchanged.
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Context Entry comicBook Series.
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State = EntityState.Unchanged.
8:00
Changing the entry state from added to unchanged
8:03
will prevent the entity from being inserted into the database.
8:08
This problem also exists for the artist collection.
8:11
While we want any of the comic book artist entities to be added,
8:15
we wouldn't want the related artist or role entities to be inadvertently added.
8:20
I'll leave adding that code as an exercise for you to complete on your own.
8:24
If you get stuck, see the teacher's notes for the solution.
8:29
Let's do some cleanup.
8:30
Go ahead and remove the code that's setting the comic book series property.
8:36
To remove the duplicate series data, let's delete the database.
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So we'll get a clean baseline copy of our data the next time that we start our app.
8:49
So, how much defensive coding is really necessary.
8:52
Well, it depends on the situation.
8:55
If you're the only developer working on the project,
8:58
then maybe this level of defensive coding is unnecessary.
9:01
Instead of writing this code, you could just ensure that you exclusively use
9:05
the foreign key properties and leave the navigation properties set to null.
9:10
But it's hard to predict how a project will evolve or
9:13
how your data access layer might be used in the future.
9:17
Given that, the best option might be to make your code as flexible and
9:21
reliable as possible.

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Entity Framework Basics

Creating Entities

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