many-to-many - w3toppers.com

How can I achieve a self-referencing many-to-many relationship on the SQLAlchemy ORM back referencing to the same attribute?

Here’s the UNION approach I hinted at on the mailing list earlier today. from sqlalchemy import Integer, Table, Column, ForeignKey, \ create_engine, String, select from sqlalchemy.orm import Session, relationship from sqlalchemy.ext.declarative import declarative_base Base= declarative_base() friendship = Table( ‘friendships’, Base.metadata, Column(‘friend_a_id’, Integer, ForeignKey(‘users.id’), primary_key=True), Column(‘friend_b_id’, Integer, ForeignKey(‘users.id’), primary_key=True) ) class User(Base): __tablename__ = ‘users’ id … Read more

how to make a composite primary key (java persistence annotation)

You’ve already had a few good answers here on how to do exactly as you ask.. For reference let me just mention the recommended way to do this in Hibernate instead, which is to use a surrogate key as primary key, and to mark business keys as NaturalId’s: Although we recommend the use of surrogate … Read more

MVC 4 – Many-to-Many relation and checkboxes

You start with two view models. The first one which represents a selected company… public class CompanySelectViewModel { public int CompanyId { get; set; } public string Name { get; set; } public bool IsSelected { get; set; } } …and the second one for the subscription to create: public class SubscriptionCreateViewModel { public int … Read more

Entity Framework Code First Many to Many Setup For Existing Tables

Remove your Essence2EssenceSet model class. If junction table contains only keys of related entities participating in many-to-many relations it is not needed to map it as entity. Also make sure that your fluent mapping of many-to-many relations specifies schema for table: mb.Entity<Essence>() .HasMany(e => e.EssenceSets) .WithMany(set => set.Essences) .Map(mc => { mc.ToTable(“Essence2EssenceSet”, “Com”); mc.MapLeftKey(“EssenceID”); mc.MapRightKey(“EssenceSetID”); … Read more

Query on a many-to-many relationship using Doctrine with Symfony2

You can write a join DQL query as below $em = $this->getContainer()->get(‘doctrine’)->getManager(); $repository = $em->getRepository(‘YourNamespaceYourBundle:User’); $query = $repository->createQueryBuilder(‘u’) ->innerJoin(‘u.groups’, ‘g’) ->where(‘g.id = :group_id’) ->setParameter(‘group_id’, 5) ->getQuery()->getResult(); Your mapping for groups property in User entity will handle join part itself you don’t have to mention the junction table in your DQL query

How to define Many-to-Many relationship through Fluent API Entity Framework?

The terms Left and Right in MapLeftKey and MapRightKey in the many-to-many mapping with Fluent API can be misunderstood and I guess your problem is caused by this misunderstanding. One might think that it means they describe the columns that are “left” and “right” in the many-to-many join table. That’s actually the case if you … Read more

Generic many-to-many relationships

You can implement this using generic relationships by manually creating the junction table between message and recipient: from django.db import models from django.contrib.contenttypes import generic from django.contrib.contenttypes.models import ContentType class Client(models.Model): city = models.CharField(max_length=16) # These aren’t required, but they’ll allow you do cool stuff # like “person.sent_messages.all()” to get all messages sent # by … Read more

How to manually load related entities in a N:N relationship?

The feature you are talking about is called Relationship Span or Relationship Fixup and indeed – as you have noticed – it does not work for many-to-many relationships. It only works if at least one end of the association has multiplicity 1 (or 0..1), i.e. it works for one-to-many or one-to-one relationships. Relationship Span relies … Read more

Many to Many Relationships not saving

It is expected that nothing happens. What you want to change or add is a relationship between the entities Service and ServiceItem. But you cannot manipulate relationships by setting the state of an entity to Modified. This only updates scalar and complex properties but no navigation properties (= relationships). (For example setting the state of … Read more

How to do many-to-many Django query to find book with 2 given authors?

Not intuitive at first but the answer is right in front of us. Book.objects.filter(author__id=1).filter(author__id=2) If you want an exact match, you could potentially further filter this result by those items that only have exactly 2 authors. Book.objects.annotate(count=Count(‘author’)).filter(author__id=1)\ .filter(author__id=13).filter(count=2) If you want exact matches dynamically, how about something like this?: def get_exact_match(model_class, m2m_field, ids): query = … Read more