Since the SERVICE_USER table is not a pure join table, but has additional functional fields (blocked), you must map it as an entity, and decompose the many to many association between User and Service into two OneToMany associations : One User has many UserServices, and one Service has many UserServices. You haven’t shown us the … Read more
The SQL DDL (data definition language) statements could look like this: CREATE TABLE product ( product_id serial PRIMARY KEY — implicit primary key constraint , product text NOT NULL , price numeric NOT NULL DEFAULT 0 ); CREATE TABLE bill ( bill_id serial PRIMARY KEY , bill text NOT NULL , billdate date NOT NULL … Read more
With a simple two-column many-to-many mapping, I see no real advantage to having a surrogate key. Having a primary key on (col1,col2) is guaranteed unique (assuming your col1 and col2 values in the referenced tables are unique) and a separate index on (col2,col1) will catch those cases where the opposite order would execute faster. The … Read more
Depending on your query needs you can put everything in the user document: {name:”Joe” ,roles:[“Admin”,”User”,”Engineer”] } To get all the Engineers, use: db.things.find( { roles : “Engineer” } ); If you want to maintain the roles in separate documents then you can include the document’s _id in the roles array instead of the name: {name:”Joe” … Read more
my_mood.interests.remove(my_interest) Django’s Relations Docs Note: you might have to get an instance of my_mood and my_interest using Django’s QuerySet API before you can execute this code.
EDIT 2017-04-29: As pointed to by some of the commenters, the JoinTable example does not need the mappedBy annotation attribute. In fact, recent versions of Hibernate refuse to start up by printing the following error: org.hibernate.AnnotationException: Associations marked as mappedBy must not define database mappings like @JoinTable or @JoinColumn Let’s pretend that you have an … Read more
Looks like everyone is answering One-to-many vs. Many-to-many: The difference between One-to-many, Many-to-one and Many-to-Many is: One-to-many vs Many-to-one is a matter of perspective. Unidirectional vs Bidirectional will not affect the mapping but will make difference on how you can access your data. In Many-to-one the many side will keep reference of the one side. … Read more
Multiple attributes can be connected with an and: GroupMember.find_or_create_by_member_id_and_group_id(4, 7) (use find_or_initialize_by if you don’t want to save the record right away) Edit: The above method is deprecated in Rails 4. The new way to do it will be: GroupMember.where(:member_id => 4, :group_id => 7).first_or_create and GroupMember.where(:member_id => 4, :group_id => 7).first_or_initialize Edit 2: Not … Read more
If you want to be able to specify ManyToMany relation without making it required just use blank=True: class Group(models.Model): … events = models.ManyToManyField(Event, blank=True)
It’s not possible to create a many-to-many relationship with a customized join table. In a many-to-many relationship EF manages the join table internally and hidden. It’s a table without an Entity class in your model. To work with such a join table with additional properties you will have to create actually two one-to-many relationships. It … Read more