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The up and
down methods in the migration class

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use the migration fluent
API to update the database.

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The migration fluent API is provided by
methods in the DB migration base class.

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The create table method is used,

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as the name suggests to create
a table in the database.

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The first parameter is
the name of the table.

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The second parameter is a lambda
that defines the table columns.

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Here we're defining a column
named Id that is of type int.

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It's configured to not allow nulls and
it's an identity column.

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And here we're defining a column
named name of type string.

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It's also configured to not allowed
nulls and has a maximum length of 100.

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Because this is a fluent API we can
dot off the create table method and

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make additional method calls to
further configure this table.

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This call to the primary key method
is configuring the Id column

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to be the tables primary key.

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Scrolling down a bit we can see
the call to the create table method for

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creating the comic book
artist database table.

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Four columns are being defined Id,
ComicBookId, ArtistId and RoleId.

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This table is an explicit bridge table

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that defines a many-to-many relationship
between comic books and artists.

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This many-to-many relationship allows
a comic book to be associated with one or

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more artists and an artist can be
associated with one or more comic books.

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For more information about defining
entity relationships in EF,

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see the teacher's notes for a link to
an earlier Treehouse course that can help.

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Here we can see that the ComicBookId,
ArtistId and RoleId columns are being

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configured as foreign keys,
using the foreign key method.

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The first parameter is the name of
the database table that contains

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the principal key.

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The second parameter is a lambda to
select the ForeignKey column that we're

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configuring.

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And the third parameter is
enabling cascadeDeletes.

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Remember, enabling cascadeDeletes will
cause rows in this table to be deleted

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when an associated
principle row was deleted.

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The index method will
create a database index

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on a specified database table column.

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Creating indexes can help improve query
performance when loading related data or

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filtering.

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For more information on database indexes,
see the teacher's notes.

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In the down method, we can see calls to
the drop versions of the methods that were

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used in the up method.

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Notice that items related to the tables

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need to be dropped first before
the tables, themselves can be dropped.

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If you try to drop a table
before its foreign keys or

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indexes were dropped, you'd get an error.

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Since we don't have an existing database,

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our entire model is represented
here in this migration class.

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In a later video, we'll make
a change to one of our in a days so

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we can see a migration that only includes
the changes that we made to our model.

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If you find an admission or
error in your migration and

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need to regenerate it you
have a couple of options.

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The first option is to rerun the add
migration command and add the force flag.

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Add-migration space,
the name of the migration

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which is Initial followed
by another space,

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-force then press Enter
to run the command.

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Since we already have the migration file
open we'll be prompted to reload it.

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The second option is to just
delete the migration code file and

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rerun the add migration
command not the force flag.

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Add-migration space Initial.

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We can do either of these options as many
times as we need to in order to get our

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migration right.

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If the migration has already been
applied to our database only to

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downgrade the database first before we
can delete and regenerate the migration.

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We'll see how to downgrade
a database in the next section.

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It's a good practice to review migrations
before you update your database and

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commit them to your
source code repository.

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If you see an error in a migration class
it typically means that you have an issue

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in your entity data model
that needs to be resolved.

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While there will be times when editing the
migration class is necessary to do we'll

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look at one of those situations
in the next section.

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You should always try to resolve
the issue by updating your model.

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That will ensure that your requirements
for your applications data

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are correctly implemented in both
the entity data model and the database.

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Most errors can be resolved by fixing
simple oversights in your entity classes

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like adding a missing navigation or
foreign key property.

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Some errors might require
you to add a data annotation

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attribute to an entity class property in
order to refine a column's data type.

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Or you might need to use the EF fluent API

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in your context classes on model creating
method to refine an entity relationship.

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Now that we have our initial
migration added to our project

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we need to update our database.

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We'll see how to do
that in the next video.