<style> .present { text-align: left; } </style> --- ###### tags: `Week 18` `W18D4` --- # Alembic Migrations & JSON with Flask ## Week 18 Day 4 --- ## Review from yesterday... - Setting up our flask apps to connect with sqlalchemy - making models - relationship attributes - 1 : 1 - 1 : Many - Many : Many - Full CRUD with posts - to_dict() methods and JSON --- ### Setting Up Flask-Migrate 1. Install packages (`alembic` and `flask-migrate`). ```bash pipenv install alembic flask-migrate ``` 2. Import `Migrate` from `flask_migrate` and use it to configure your migrations (in the file where you are defining your Flask application). ```python= # add this to the top of the file with the imports from flask_migrate import Migrate # this goes after your app is instantiated Migrate(app, db) ``` --- ### Setting Up Flask-Migrate 3. Initialize your migrations repo. ```bash flask db init ``` 4. Set up the correct revision file name in the alembic.ini ```bash= file_template = %%(year)d%%(month).2d%%(day).2d_%%(hour).2d%%(minute).2d%%(second).2d_%%(slug)s ``` 5. Create a migration file. ```bash flask db migrate ``` 6. Apply your migrations to your database. ```bash flask db upgrade ``` --- ### Workflow When you make changes to your models: 7. Run migrate to get a new migration script. ```bash flask db migrate ``` 8. Run upgrade to apply the changes. ```bash flask db upgrade ``` --- ### Alembic won't detect every change... By default, Alembic only detects simple changes like new or dropped columns/models. It does not detect smaller changes to existing columns or models. There are a few ways you can configure Alembic to detect these sorts of changes. --- ### Changing a Table Name If you want to change the name of an existing table, use the `op.rename_table()` method in the migration script: ```python= op.rename_table('old_name', 'new_name') ``` To do so, first generate the migration file and then add the above function call to the `upgrade()` function. --- ### Changing a Column Name If you want to change the name of an existing column, use the `op.alter_column()` method with the kwarg `new_column_name` in the migration script: ```python= op.alter_column('table_name', 'old_name', new_column_name='new_name') ``` To do so, first generate the migration file and then alter the `upgrade()` function with the above function call. --- ### Changing a Column Constraint To configure Alembic to detect changes to an existing column's constraints, use the `compare_type=True` keyword argument. In the `migrations/env.py` file, locate the final `context.configure()` call and add `compare_type=True` to the keyword arguments. --- ### Downgrading Downgrading shouldn't be a very common part of your workflow. If you want to downgrade a migration, you can use the `downgrade` command. To undo a single migration: ```bash flask db downgrade ``` --- ### Model Instances vs. Dictionaries Recall that, in JavaScript, we typically use objects for two purposes: as a collection of key/value pairs, and also as a collection of methods and properties. In Python, key/value pairs in dictionaries are not the same as attributes on objects (a.k.a, classes). ```python= book = Book.query.get(1) print(book) print(book.title) ``` If you try to send a model instance object from your server to your frontend, the frontend client won't know what to do with it. If we send a dictionary instead, Flask will automatically convert it to JSON so it can be understood by the client. --- ### Returning JSON from Python We need to convert our database objects into dictionaries so that we can use our Flask back-end as an API. ```python= book = Book.query.get(1) book_dict = { "id": book.id, "title": book.title, } print(book_dict) # Flask can automatically convert this to JSON ``` --- ### Writing a `to_dict()` method We can make our code `DRY`er by adding a method to our model classes to convert model instances to dictionaries. That way we don't have to rewrite the same code on different routes when we send data to our front-end. ```python= class Book(db.Model): __tablename__ = "books" id = db.Column(db.Integer, primary_key=True) title = db.Column(db.String(50), nullable=False) # other columns, methods, etc... def to_dict(self): return { "id": self.id, "title": self.title, } ``` --- ### Project time! Today's project is Package Tracker. You and your pair will build another data-driven Flask/SQLAlchemy application to track shipped packages. The project will ask you to use Flask-Migrate to migrate your data.