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The `Up` and `Down` methods in the migration class use the Migration Fluent API to update the database. Let's take a look at some of the methods in the DbMigration base class that make up the Migration Fluent API.
Follow Along
To follow along commiting your changes to this course, you'll need to fork the dotnet-ef-migrations repo. Then you can clone, commit, and push your changes to your fork like this:
git clone <your-fork>
cd dotnet-ef-migrations
git checkout tags/v1.5 -b the-migration-fluent-api
Additional Learning
For more information on how to define EF entity relationships, see the following Treehouse course that covers the basics of EF.
See the following article for more information about database indexes.
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The up and
down methods in the migration class
0:00
use the migration fluent
API to update the database.
0:02
The migration fluent API is provided by
methods in the DB migration base class.
0:07
The create table method is used,
0:14
as the name suggests to create
a table in the database.
0:16
The first parameter is
the name of the table.
0:22
The second parameter is a lambda
that defines the table columns.
0:26
Here we're defining a column
named Id that is of type int.
0:32
It's configured to not allow nulls and
it's an identity column.
0:38
And here we're defining a column
named name of type string.
0:45
It's also configured to not allowed
nulls and has a maximum length of 100.
0:52
Because this is a fluent API we can
dot off the create table method and
0:58
make additional method calls to
further configure this table.
1:03
This call to the primary key method
is configuring the Id column
1:08
to be the tables primary key.
1:11
Scrolling down a bit we can see
the call to the create table method for
1:14
creating the comic book
artist database table.
1:18
Four columns are being defined Id,
ComicBookId, ArtistId and RoleId.
1:21
This table is an explicit bridge table
1:28
that defines a many-to-many relationship
between comic books and artists.
1:31
This many-to-many relationship allows
a comic book to be associated with one or
1:35
more artists and an artist can be
associated with one or more comic books.
1:40
For more information about defining
entity relationships in EF,
1:46
see the teacher's notes for a link to
an earlier Treehouse course that can help.
1:50
Here we can see that the ComicBookId,
ArtistId and RoleId columns are being
1:56
configured as foreign keys,
using the foreign key method.
2:01
The first parameter is the name of
the database table that contains
2:09
the principal key.
2:12
The second parameter is a lambda to
select the ForeignKey column that we're
2:15
configuring.
2:19
And the third parameter is
enabling cascadeDeletes.
2:22
Remember, enabling cascadeDeletes will
cause rows in this table to be deleted
2:26
when an associated
principle row was deleted.
2:30
The index method will
create a database index
2:35
on a specified database table column.
2:38
Creating indexes can help improve query
performance when loading related data or
2:42
filtering.
2:47
For more information on database indexes,
see the teacher's notes.
2:48
In the down method, we can see calls to
the drop versions of the methods that were
2:57
used in the up method.
3:02
Notice that items related to the tables
3:05
need to be dropped first before
the tables, themselves can be dropped.
3:08
If you try to drop a table
before its foreign keys or
3:14
indexes were dropped, you'd get an error.
3:17
Since we don't have an existing database,
3:20
our entire model is represented
here in this migration class.
3:22
In a later video, we'll make
a change to one of our in a days so
3:27
we can see a migration that only includes
the changes that we made to our model.
3:30
If you find an admission or
error in your migration and
3:35
need to regenerate it you
have a couple of options.
3:38
The first option is to rerun the add
migration command and add the force flag.
3:42
Add-migration space,
the name of the migration
3:50
which is Initial followed
by another space,
3:55
-force then press Enter
to run the command.
4:00
Since we already have the migration file
open we'll be prompted to reload it.
4:08
The second option is to just
delete the migration code file and
4:12
rerun the add migration
command not the force flag.
4:17
Add-migration space Initial.
4:24
We can do either of these options as many
times as we need to in order to get our
4:34
migration right.
4:39
If the migration has already been
applied to our database only to
4:41
downgrade the database first before we
can delete and regenerate the migration.
4:45
We'll see how to downgrade
a database in the next section.
4:50
It's a good practice to review migrations
before you update your database and
4:54
commit them to your
source code repository.
4:58
If you see an error in a migration class
it typically means that you have an issue
5:01
in your entity data model
that needs to be resolved.
5:05
While there will be times when editing the
migration class is necessary to do we'll
5:08
look at one of those situations
in the next section.
5:13
You should always try to resolve
the issue by updating your model.
5:16
That will ensure that your requirements
for your applications data
5:20
are correctly implemented in both
the entity data model and the database.
5:24
Most errors can be resolved by fixing
simple oversights in your entity classes
5:29
like adding a missing navigation or
foreign key property.
5:34
Some errors might require
you to add a data annotation
5:38
attribute to an entity class property in
order to refine a column's data type.
5:41
Or you might need to use the EF fluent API
5:47
in your context classes on model creating
method to refine an entity relationship.
5:50
Now that we have our initial
migration added to our project
5:56
we need to update our database.
6:00
We'll see how to do
that in the next video.
6:02
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