many-to-many

问题 This is my model, Players and Clubs. As a Club can have many players and a player can have many clubs (in its carrer), I used a many-to-many relationship: class Club(db.Model): name = db.StringProperty() link = db.StringProperty() class Player(db.Model): name = db.StringProperty() link = db.LinkProperty() class ClubHasPlayer(db.Model): club = db.ReferenceProperty(Club, required=True, collection_name='club_players') player = db.ReferenceProperty(Player, required=True, collection_name='player

问题 I´m trying to map an existing database to Hibernate using annotations. The problem is as follows. 'organization' and 'organization_roles' has a many to many relationship which is resolved in a table 'organization_has_roles' using the two primary keys as a composite primary key in the join table. Next there is a 'users' table. Users can have a single role in multiple organizations. This is again resolved in a join table named 'users_has_organizations_and_role' which has a composite primary key

问题 I followed the suggestions from user ehp in order to build a lightweight messaging-system: https://stackoverflow.com/a/18717864/1084315 Users: id | username Messages: id | from | content user_messages: user_id | message_id class User extends Eloquent { public function messages() { return $this->belongsToMany('Message'); } public function sent_messages() { return $this->hasMany('Messages', 'from'); } } class Message extends Eloquent { public function from() { return $this->belongsTo('User',

问题 I am building a recipe manager as a first rails app. I have a many-to-many, nested set of models based on this quite nice ERD. Ideally, I would like to create a form that allows me to create the recipe in a single form. Also, I would like the user to be able to write/paste the ingredients and steps into a single text field, respectively. In the code below, there is a virtual attribute that parses the cr separated lists in the form to attempt this. I keep getting a "readonly" has_many through

问题 What's the right way to model many-to-many relationships in Cassandra (using 3.10 at the moment)? From what answers I was able to find, denormalization into two relationship tables is suggested (as in here, for example: Modeling many-to-many relations in Cassandra 2 with CQL3). But there are problems with that on deletes, and those answers are so sparse they do not mention any details on that. Suppose we have the following tables: CREATE TABLE foo ( key UUID PRIMARY KEY, content TEXT ) CREATE

问题 I've had an interesting problem with a database design whose solution is not yet satisfactory. I have a table of Stations with the following fields: - key_station (Primary key) - station_name - info Then I have a table of Positions : - key_position (Primary key) - position_number - info - key_station (Foreign key to Stations) And table of Setups : - key_setup (Primary key) - setup_number - info - key_station (Foreign key to Stations) Now, the Positions table lists all the slot numbers of a

问题 I have a many-to-many relationship User to Lists User model was created with devise. For some reason I cant access .lists on any of the users. returns undefined method `to_sym' for nil:NilClass. after doing some digging im pretty sure ive found the problem in _reflect_on_associationactiverecord (4.1.6) lib/active_record/reflection.rb def _reflect_on_association(association) #:nodoc: _reflections[association.to_sym] end association is being passed in as nil. Has anyone run into this problem

问题 Here is my problem (a bit simplified) : I've got the following models : class User has_many :group_users has_many :groups, through: :group_users class Group has_many :group_users has_many :users, through: :group_users class GroupUser belongs_to :group belongs_to :user scope :belonging_to_group ->(group) {where(group_id = group.id)} I would like to scope users that are not in one specific group, let's say veggies for examples, something that would begin like this : scope :not_in_group, ->

问题 I followed the docs down to the letter and can't get the M2M autocomplete lookup to work in Grappelli. #models.py #main model class Entry(models.Model): title = models.CharField(max_length=60) content = models.TextField() keywords = models.ManyToManyField(Keyword, blank=True) #model I want to be searched through while typing in the autocomplete field class Keyword(models.Model): name = models.CharField(max_length=30) @staticmethod def autocomplete_search_field(): return ('id__iexact', 'name_

问题 I am coming from the relational database world (Rails / PostgreSQL) and transitioning to the NoSQL world (Meteor / MongoDB), so I am learning about denormalization, embedding and true links. It seems that, in many cases, choosing between various database schemas comes down to the number of documents that will be "related" to each others. In this video series, the author distinguishes: one-to-many relationships from one-to-few relationships many-to-many relationships from few-to-few