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Many to Many relationships.
An interesting read on Security.
Entity Relationships are the n-to-n associations between two entities, or tables in a database.
A Team Member can be a part of multiple Projects, and a Project can have many Team Members working on it.
Each table has their own Primary Key.
To form the relationship, the local table will have a "Foreign Key" that points to the other table's Primary Key.
When setting up tables for many-to-many, SQL commands can be used, or a UI like PgAdmin4 or SQL Server Management Studio (choose your poison).
The Classes in code must be set as Entities using @Entity annotation at the Class definition.
Optionally, @Table(name="table_name") can be added at the Class definition to point to a specific table, which is useful when the could otherwise be a naming clash in the database.
Use the annotation @ManyToMany(cascade={CascadeType.NNN}) prior to joining tables within the Class.
Use the annotation @JoinTable(...) to define the joining SQL commands within the parentheses.
A HashSet is defined as the pluralized Property of the Class e.g. Set<Project> projects = new HashSet<>();
On the other side (the foreign Entity) the @Entity and @Table... and @ManyToMany(mappedBy="previous_table") annotations are used, and a HashSet is definied as the pluralized Property of the foreign Class e.g. private Set<TeamMember> teamMembers = new HashSet<>();
[ ] Use @ManyToMany on both Entities to create the relationship.
[ ] The 'owning' side of the relationship is @JoinTable annotation Class.
[ ] The Join code defines how the tables are related following @JoinTable annotation.
[ ] Use the 'mappedBy' attributes indicates this Class is the foreign table.
See the unittest example at Baeldung.com
Very fun article.
Key takeaways:
Spring Hibernate Many to Many Annotation Tutorial
Harvard Scholar University - USENIX ";Login:logout" Article This World of Ours
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