many-to-many

问题 This is a strange scenario born of the fact that I am working with a legacy system who's structure I cannot change. The scenario is that I have a Element table that contains various data about an element with to important fields "type" and a "value". Various combinations of type and value are categorised. For instance, all Elements of type 2 and value "green" belong to category 1. I'm building a user interface in to the legacy system that lets users define these categories given a type and

问题 i have a Users and Tags table,and also a user_tag_xref table that holds the many to many relationship.now netbeans generates the entity classes for me (using eclipselink) below is the entity mapping relationship on User class @ManyToMany(mappedBy = "usersList") private List<Tags> tagsList; on Tags class @JoinTable(name = "USERS_TAG_XREF", joinColumns = { @JoinColumn(name = "TAG_ID", referencedColumnName = "TAG_ID")}, inverseJoinColumns = { @JoinColumn(name = "USER_ID", referencedColumnName =

问题 Been reading the tutorial How to handle a Many-to-Many relationship with PHP and MySQL . In this question I refer to the "Database schema" section which states the following rules: This new table must be constructed to allow the following: * It must have a column which links back to table 'A'. * It must have a column which links back to table 'B'. * It must allow no more than one row to exist for any combination of rows from table 'A' and table 'B'. * It must have a primary key. Now it's

问题 I try to model many to many relationship between Car and Employee where the info about when the car was used are held. So I need a class inbetween which will hold those additional attributes. The SSCCE: I have Person class: public class Person { public int Id { get; set}; } and a class Employee which derives from Person : public class Employee : Person { public virtual ICollection<EmployeeCar> EmployeeCar { get; set; } } and a association class EmployeeCar which holds attributes dateFrom ,

问题 I have two tables Country and Region. A Region is a set of one or several Countries. Some Countries are in no Region. How should I represent this in a relational database? I thought of the two following possibilities: Country has a Edition column that includes either null or the Edition that it belongs to. My problem with this: I have been taught nulls are evil in a database. Edition has a Countries column that is an array of Country. My problem with this: I have been taught arrays are evil

问题 in Teams#model class Team < ActiveRecord::Base has_many :postteams has_many :posts, :through => :postteams end in Post#model class Post < ActiveRecord::Base has_many :postteams has_many :teams, :through => :postteams accepts_nested_attributes_for :postteams end In Postteam Model class Postteam < ActiveRecord::Base belongs_to :post belongs_to :team end Through these relations, i finally succeed to make post with these multiple teams and also save these teams in database, like this 1 here is my

问题 Tied myself a bit up in knots in the development of a relatively complicated relational database. Purpose of database is to allow managers to arrange meetings with representatives (or groups of representatives) from particular companies. Naturally the table, outlining the details relating to arranged meetings, is of utmost importance. Unfortunately most of its information is nigh impossible for users to parse as the fields are merely returning the numerical fields of other tables. So really

问题 I'm having a problem with Django's generated SQL from the ORM. Cartons have a many to many relationship with Shipments through cartons_shipments . I'm looking to exclude Shipments where they have at least one INBOUND Carton that has a status in ['TRANSIT', 'DELIVERED', 'FAILURE'] . But I was not getting the results I expected so I turned on the SQL logging. return Shipment.objects.filter( ... # other filtering # does not have any inbound cartons in_transit/delivered/failed ~Q( Q(cartons__type

问题 So I'm trying to learn rails and I don't want to cheat just yet. post model: class Post < ActiveRecord::Base has_many :features has_many :tags, :through => :features end tag model: class Tag < ActiveRecord::Base has_many :features has_many :posts, :through => :features end join table: class Feature < ActiveRecord::Base belongs_to :tag belongs_to :post end I already know how to associate posts with tags by doing: Post.find_by_id(1) << Tag.first Now, I'm stuck on searching for posts with

问题 I'm mapping a proprietary database to Hibernate for use with Spring. In it, there are a couple of jointables that, for entity A and entity B have the following schema: CREATE TABLE AjoinB ( idA int not null, idB int not null, groupEnum enum ('groupC', 'groupD', 'groupE'), primary key(idA, idB, groupEnum) ); As you can see, this indicates that there can be multiple A-B relationships that put them in different groups. I'd like to end up with, first line for entity A and second for entity B, the