Sequelize is an ORM (Object-Relational Mapping) that makes it easy to create and manage relationships between database tables in a relational database. It is a popular choice for developers who need to interact with multiple databases, as it supports a wide variety of database systems, including MySQL, MariaDB, PostgreSQL, SQLite, and MSSQL.
The most common way to create a relationship between tables in a database is to use associations, which are a type of join. A join is a SQL query that connects two tables together, often using foreign keys. Associations are a type of join that allow you to define the relationship between two tables in terms of an object model. This makes it easier to work with data in an object-oriented environment.
For example, if you have a table of users, and each user has a list of orders, you can create an association between the two tables to define the relationship between the two. This allows you to easily retrieve the orders for a user, or the user for an order.
Sequelize makes it easy to create and manage associations. It provides a number of different types of associations, including one-to-one, one-to-many, many-to-one, and many-to-many. Each of these types is defined by the type of relationship between the two tables. For example, a one-to-one relationship is a relationship where one row in one table is related to one row in another table, whereas a one-to-many relationship is a relationship where one row in one table is related to multiple rows in another table.
Sequelize also provides a number of different methods to define associations, including defining the association in the model definition, or by using the associate() method. The associate() method allows you to define the association in code, which makes it easier to maintain and debug.
Once the association is defined, Sequelize makes it easy to access the associated data. For example, if you have a table of users and orders, you can use the findAll() method to get a list of all orders for a user. You can also use the include() method to get the associated data for a particular order, such as the user who placed the order.
Sequelize also provides a number of ways to customize the behavior of associations. For example, you can define the cascade delete behavior, which will automatically delete associated data when the parent is deleted. You can also specify the fetch type, which determines how associated data is retrieved from the database.
Overall, Sequelize is a powerful and easy-to-use ORM that makes it easy to create and manage associations between database tables. Its various methods for defining associations, as well as its ability to customize the behavior of associations, make it an ideal choice for developers who need to interact with multiple databases.