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- [Instructor] While the Django models

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define the expected structure of our database,

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migrations are responsible

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for creating the necessary scripts

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to change this structure through time

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as we update our code to change our models.

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There are several cases in which a migration is needed.

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When a new model is created,

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a migration creates the corresponding database table.

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Migrations are also needed

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when a field is added or removed from an existing model,

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or, when attributes of a field have changed.

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All of these changes to a model's file

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need a corresponding change to the database,

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and for these purposes migrations need to be created,

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and then run.

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The first migration created for a new Django app

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will create tables for the models that are defined.

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These migrations are called initial migrations.

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We've defined models for our app,

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but we have yet to create these initial migrations

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and we'll tackle this in a moment.

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The commands for working with migrations are

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makemigrations, showmigrations and migrate.

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The makemigrations command generates migration files.

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It reads the current model's file

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and inspects the current state of the database

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to determine what changes need to be made

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to make the database structure match the model's file.

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Those files are placed in the migration's folder

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of the corresponding app,

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and are automatically numbered, starting with one.

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Therefore, our initial migration will be named

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starting with one, and it will be stored

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in the adoption/migrations folder.

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The migrate command runs all the generated migrations

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that have not yet run.

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We can also run migrations for a specific app

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to a specific number of migration,

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by using the migrate command with an app name and a number.

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As an example, we could use migrate adoptions one

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to migrate to the first migration for the adoptions app.

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When a migration has been created, but not yet run,

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we call this an unapplied migration.

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This is a common source of errors during development,

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especially when collaborating with other developers.

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With this in mind, be sure that when working on a team,

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you coordinate carefully who is changing which model,

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and to look for new migration files

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when pulling in code changes.

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Now, let's take a moment to look at generating

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and running migrations for our Django project.

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I've opened a terminal,

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and now I'll navigate to the folder

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where the manage.py file is.

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For me, this will be on the desktop,

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then my learning Django project folder,

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and then the wisdompets folder.

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From this folder,

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I'll use ls to ensure I'm in the right place

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and that the manage.py file is here.

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Remember, on Windows, you'll need to use D-I-R.

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Now, to make our migrations, I'll type python3,

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space, manage.py, and then makemigrations,

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all one word, lower case.

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On Windows, you'll need to use python instead of phython3.

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From the output we see,

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we can see that the adoptions app

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now has migrations for the pet and vaccine models.

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The migration we just generated

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will create these models for the first time,

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so this will be an initial migration.

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Now, let's see that the migration was created

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by looking inside the adoptions folder

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and inspecting the migrations folder with ls,

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or D-I-R on Windows.

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If you want, you could also open this file in a text editor.

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I'll just type ls space adoptions/migrations.

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I can see now that there's a new file,

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and it's called 0001, underscore initial.py.

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To see which migrations exist for our app,

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and which ones have not yet run,

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I can use this show migrations command.

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So now, I'll type pyhthon3, space, manage.py,

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and showmigrations.

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There are several migrations listed,

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and they are grouped by the corresponding app name,

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alphabetically.

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Several of these are default Django apps,

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such as admin, auth and so on.

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In the second group, we can see our adoptions app.

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The default apps come with models and migrations.

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So, those migrations also appear in this list.

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The square braces on the left side of this output

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with an empty space indicates that these migrations

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have not yet been applied.

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To apply our migrations,

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we will now run phython3, manage.py and migrate.

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From the output,

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we can see that all of the migrations

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were applied successfully.

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With this in place, let's run show migrations again

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to verify our results.

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I'll type phython3, manage.py and show migrations.

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Now, the square braces on the left have an X inside,

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indicating that each migration has been applied.

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Now that we've run all of our migrations,

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let's see what the structure of our database looks like.

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I've downloaded a database browser tool

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from sqlitebrowser.org,

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which is an open source project

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that's available on most platforms.

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This tool will allow us to inspect SQLite databases

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and see their structure and contents.

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First, inside the tool,

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I'll open the db.sqlite3 file for our project.

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Since we created models called pet and vaccine

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inside of our adoptions app,

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the tables are named adoptions underscore pet,

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and adoptions underscore vaccine.

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We also have a third table called adoptions underscore pet,

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underscore vaccinations.

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This third table is used for storing the mini-to-mini field

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for pets and vaccines.

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To look inside the adoptions pet table,

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I'll click the unfold button here on the left side.

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Inside this table, we can see that there are several fields

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including ID, name, submitter and so on.

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ID is an automatically generated field

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for all tables that Django manages.

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The remaining fields match those

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that are defined in our model.

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After creating and running all of the migrations,

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we've now confirmed that they are applied successfully

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and we're ready to use our database.