Trying to understand relationships in SQLAlchemy : flask
This cheat sheet sticks to parts of the ORM (Object Relational Mapper) layer,and aims to be a reference not a tutorial. That said, if SQLAlchemy makes leveraging and examining those relationships pretty straight forward. Do you know what is 'The effect of geography lesson'? Now, how to connect objects with pythonic ways, when to use association, New-style classes were introduced in Python to unify the concepts of class and type. In SQLAlchemy you have an object called the “session”. As such you can directly declare relationships and attributes in the class itself.
The best examples of using mixins properly — we can see in Sqlalchemy, which is one of the most popular object relational mapper and sql toolkit for Python. For example, when we want to create models, which will be connected with tables that have similar names, and we want to create for each table id as a pk, we can write our mixin and after this, we create MyModel class which will be subclass of base class and also subclass of mymixin, we get class like following: This is our class structure: Next we create mixin RefTargetMixin and we can add this mixin as a parent class for our foo and bar classes.
Final result will be like this: There are also more advanced situations when sqlalchemy uses mixins and you can read the documentation to see another examples. So in summary we can say that mixins are best when we want to provide different optional features for a class. So, usually mixins are used very often in frameworks and by popular toolkits. Association And what about associations? For example, because there is very popular situation in oop, when you want a banana, but what you get is a gorilla holding the banana.
You wanted a banana but what you got was a gorilla holding the banana and the entire jungle. This is exactly where we need the concept of association. In object-oriented programming, association defines a relationship between classes of objects that allows one object instance to cause another to perform an action on its behalf. And we will see how it actually does it. For example, if we have Class of conference and we want to instantiate python conference class from it, we will do the following.
Composition is a restricted form of Aggregation in which two classes are highly dependent on each other.
SQLAlchemy and You
For example human and body parts, we need body parts to live and body parts need our body to survive. This is convenient because we can add a column or a relationship to the model, and it will show up in the serialized JSON without additional coding.
On the other hand, this creates an obligation to do a lot of sub-loads, which we established above as being potentially burdensome. The solution here is easy. In our case, as our model got more and more complex, maintaining a blacklist became hard to manage, so we switched to a whitelist. Here is an abridged version of our base order query. This is helpful for discovering relationships that might throw an exception later when the object becomes detached from the session.
More on this in a bit. The query that gets emitted is pretty gnarly. Furthermore at the end of an HTTP request in Flask the extension will automatically destroy the session and discarding uncommitted changes. With that, it looks a lot closer to Django: The scoped session automatically provides a proxy that always point to the current active session.
- SQLAlchemy 1.3 Documentation
- Speed up with eager loading in SQLAlchemy
- Websites built with Flask
The Declarative Extension For a long time SQLAlchemy made you declare table objects first and then separately create the classes and map those together. This has the advantage over just subclassing some magical baseclass that you can map already existing classes to things in the database.
How to connect objects with each other in different situations with Pythonic ways
The downside always was however that you had to declare multiple objects and the common case was unnecessarily complex. SQLAlchemy since introduced the declarative base. It's a extension module shipped with SQLAlchemy that provides a function which creates a brand new baseclass which you can also customize which does metaclass magic very similar to Django. As such you can directly declare relationships and attributes in the class itself. There are still some differences though: Relationships are not magically created for you, you have to be expicit.
The same is true with foreign keys. Primary keys are not automatically generated for you for the simple reason that SQLAlchemy supports more than one primary key type. If you want one chosen by default, you can provide a baseclass that implements that. The table name has to be set explicitly. Again you can customize the baseclass to derive the table name from the class name if you like.
A SQLAlchemy Cheat Sheet
To get this baseclass you basically just need this: Flask-SQLAlchemy for instance sets the lowercase version of the class as default tablename unless overridden. Many-to-One Relationships In Django this is straightforward: First we need to declare the foreign key. It has to have the same type as the primary key of the table we want to point to and additionally the column needs to be given a ForeignKey instance with the first argument being the dotted name to the column referenced.
Note that this is the table name, not the class name.Flask & Flask-SQLAlchemy Model Relationships
The relationship is then declared on Car with relationship. The first argument is a class or the name of a class we want to have the relationship with. By default it will try to find a valid join condition automatically. If it does not, you can explicitly provide one as a string or real expression: It will attach a cars property on the manufacturer.