One to many relationship sqlalchemy query

python - Query One to Many Relationship SQLAlchemy - Stack Overflow

one to many relationship sqlalchemy query

In order to define a One to Many Relationships with Flask-SQLAlchemy you need “lazy='dynamic'”: This will return a query object which you can refine further. Deployment. Use SQL with Flask to query Postgres Databases. be able to: Create a one-to-many association using Flask-SQLAlchemy and Flask Migrate. SQLAlchemy is a deep and powerful thing made up of many layers. #fetch everything lBooks = #returns a Query object. for Note that the relationship is configured in both directions in one line.

one to many relationship sqlalchemy query

Many-to-Many A many-to-many relationship is a bit more complex. As an example, consider a database that has students and teachers. I can say that a student has many teachers, and a teacher has many students. It's like two overlapped one-to-many relationships from both ends. For a relationship of this type I should be able to query the database and obtain the list of teachers that teach a given student, and the list of students in a teacher's class.

This is actually non-trivial to represent in a relational database, as it cannot be done by adding foreign keys to the existing tables. The representation of a many-to-many relationship requires the use of an auxiliary table called an association table. Here is how the database would look for the students and teachers example: While it may not seem obvious at first, the association table with its two foreign keys is able to efficiently answer all the queries about the relationship.

Many-to-One and One-to-One A many-to-one is similar to a one-to-many relationship. The difference is that this relationship is looked at from the "many" side.

{ One To Many Associations. }

A one-to-one relationship is a special case of a one-to-many. The representation is similar, but a constraint is added to the database to prevent the "many" side to have more than one link. While there are cases in which this type of relationship is useful, it isn't as common as the other types. Representing Followers Looking at the summary of all the relationship types, it is easy to determine that the proper data model to track followers is the many-to-many relationship, because a user follows many users, and a user has many followers.

But there is a twist.

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In the students and teachers example I had two entities that were related through the many-to-many relationship. But in the case of followers, I have users following other users, so there is just users. So what is the second entity of the many-to-many relationship? The second entity of the relationship is also the users. A relationship in which instances of a class are linked to other instances of the same class is called a self-referential relationship, and that is exactly what I have here.

Creating One-To-Many Relationships in Flask-SQLAlchemy

Here is a diagram of the self-referential many-to-many relationship that keeps track of followers: The followers table is the association table of the relationship. The foreign keys in this table are both pointing at entries in the user table, since it is linking users to users.

Each record in this table represents one link between a follower user and a followed user. Like the students and teachers example, a setup like this one allows the database to answer all the questions about followed and follower users that I will ever need. Database Model Representation Let's add followers to the database first.

Here is the followers association table: Note that I am not declaring this table as a model, like I did for the users and posts tables. Since this is an auxiliary table that has no data other than the foreign keys, I created it without an associated model class. Now I can declare the many-to-many relationship in the users table: Many-to-many followers relationship class User UserMixin, db.

Like I did for the posts one-to-many relationship, I'm using the db.

one to many relationship sqlalchemy query

This relationship links User instances to other User instances, so as a convention let's say that for a pair of users linked by this relationship, the left side user is following the right side user. I'm defining the relationship as seen from the left side user with the name followed, because when I query this relationship from the left side I will get the list of followed users i. Let's examine all the arguments to the db.

one to many relationship sqlalchemy query

Since this is a self-referential relationship, I have to use the same class on both sides. From the left side, the relationship is named followed, so from the right side I am going to use the name followers to represent all the left side users that are linked to the target user in the right side.

The additional lazy argument indicates the execution mode for this query. A mode of dynamic sets up the query to not run until specifically requested, which is also how I set up the posts one-to-many relationship. Don't worry if this is hard to understand. I will show you how to work with these queries in a moment, and then everything will become clearer. The changes to the database need to be recorded in a new database migration: For example, if I had two users stored in user1 and user2 variables, I can make the first follow the second with this simple statement: Instead, I'm going to implement the "follow" and "unfollow" functionality as methods in the User model.

It is always best to move the application logic away from view functions and into models or other auxiliary classes or modules, because as you will see later in this chapter, that makes unit testing much easier. Here's what that might look like you can put this in your app. Though if you want, you can define a repr instance method on excuses so that you get more helpful messages when you look at an excuse in the terminal. At this point, your app. Remember, we only need to init once: You should see something like this: It's only if you execute the excuses query e.

one to many relationship sqlalchemy query

For comparison, if you change the lazy value to 'select' and do the same thing, you should see that the query against the excuses table gets run as soon as you look at elie. The excuses attribute automatically contains the data, not just a query object. What about 'joined' and 'subquery'?

Declaring Models — Flask-SQLAlchemy Documentation ()

With 'joined', the query to excuses is run immediately when we get the student, rather than being delayed until we look at elie. In this case, only one query is made to the database, it's just a bit more complicated, since it gets the student and all of that student's excuses at the same time. If you'd like to learn more about joins vs. For now, we won't emphasis this distinction much - none of the later examples will ever set lazy to be 'subquery'. Screencast If you'd like to see an example of building an application with a 1: M association and migrations, feel free to watch the screencast below.