one-to-many

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 I have a problem with NHibernate. What i am trying to to is very simple: I have two Classes. UserTicket and UserData. A UsertTicket has some UserData and a UserData belongs to one UserTicket: public class UserData{ public virtual int Id { get; set; } public virtual String PDF_Path { get; set; } } public class UserTicket { public virtual int Ticketnr { get; set; } public virtual IList<UserData> UserData { get; set; } } And here the mappig xml: <class name="UserTicket" table="UserTicket"> <id

问题 I'm Using Grails 2.3.11 . I have an issue in saving one-to-many dynamic forms. I referred to this question Grails one-to-many databinding with dynamic forms I'm posting as a new question since I could not add comment to the above referred post. So don't mark it as duplicate. I created dynamic form based on this blog http://omarello.com/2010/08/grails-one-to-many-dynamic-forms/ The same worked for me in my previous application which using Grails 2.2.3 Domain Classes: Contact Class package blog

问题 I have two entity objects (A and B) that have a One-To-Many relationship. I am using JPA (Hibernate) to join these tables and query them for a specific result set, but the criteria I specify for the child table (B) are not applied when fetching my results. This is how I have defined the query: CriteriaBuilder builder = entityManager.getCriteriaBuilder(); CriteriaQuery<A> query = builder.createQuery(A.class); Root<A> a = query.from(A.class); Join<A, B> abJoined = a.join(A_.b); query.distinct

问题 Primary Entities: Client Guest Reservation RoomAssignment I want to implement a multiple room reservation database design. First, I want to explain first the concept: The Client is the one who acquires a reservation. The Client can only have 1 reservation at a time The Client can reserve multiple rooms. The Guest is the one who is assigned into a specific room. So for the Table: Client (client_id(PK), Name) Guest (guest_id(PK), Name) Reservation (reservation_id(PK), client_id(FK), roomAss_id

问题 I use Symfony2 and twig templating. Think of the Q&A exact same as stackoverflow. There are list of questions with the count of score, answers, views and so on. How to count the answers of the qeustions inside loop in twig? There are OneToMany relation between Question and Answer tables. {% for question in questions %} <li>{{ question.score }}</li> <li>{# there should be the count // count($question->getAnswers()) #}</li> <li>{{ question.view }}</li> {% endfor %} Or if there is any better way

问题 How to configure multiple one to many relationship to the same entity? Consider two classes Boy and Toy. They look as shown below class Boy { @OneToMany(mappedBy='ownedBy',fetch=FetchType.EAGER, cascade=CascadeType.ALL) private List<Toy> bikes; ... @OneToMany(mappedBy='ownedBy',fetch=FetchType.EAGER, cascade=CascadeType.ALL) private List<Toy> cars; ... class Toy { @ManyToOne private Boy ownedBy; ... I know that this config is wrong. But I dont know what needs to be configured here. What

问题 I am trying to configure this @OneToMany and @ManyToOne relationship but it's simply not working, not sure why. I have done this before on other projects but somehow it's not working with my current configuration, here's the code: public class Parent { @OneToMany(mappedBy = "ex", fetch= FetchType.LAZY, cascade=CascadeType.ALL) private List<Child> myChilds; public List<Child> getMyChilds() { return myChilds; } } public class Child { @Id @ManyToOne(fetch=FetchType.LAZY) private Parent ex; @Id

问题 I have two classes as follows. class Donkey { public Guid Id { get; set; } } class Monkey { public Guid Id { get; set; } public Donkey Donkey { get; set; } } Now I want to configure the schema so that the relation is set in the database. Using Fluent API, I'll go something like this. protected override void OnModelCreating(DbModelBuilder model) { base.OnModelCreating(model); model.HasDefaultSchema("dbo"); ... model.Entity<Monkey> .HasRequired(_ => _.Donkey) .WithMany(_ => _.Monkeys) .Map(_ =>

问题 I have created a nested loop using Grails tags but not getting the output that I am expecting. I am expecting a list of links nested within another set of links. I am close but the nested links are being displayed as one big list, not multiple links. I have two domains with a one-to-many relationship. My controller is currently dynamic. writen in Grails 2.3.3 Here are my two domains class Committees { String committeeName String description static belongsTo = [hospital:Hospital] static