entity-relationship

问题 I have an entity class, correctly defined in a managed context. This class has a one-to-many relationship with another class. Xcode 4 graphic facilities correctly created the derived classes, and the relationship is represented by a NSSet. I am wondering how the creation of the relation classes is managed. I mean, for creating the main entity I am using the NSManagedObject *newEntity = [NSEntityDescription insertNewObjectForEntityForName:@"EntityName" inManagedObjectContext:context]; But what

问题 what is the difference between owned one to many relationship and owned one to many bidirectional relationship i read the article below but i don't understand it. Article 回答1: The owned one to many bidirectional relationship just means that the children have a reference to the parent. For example, the child below can access the parent via persistentUser. If persistentUser didn't exist in the PersistentLogin class then it would not be bidirectional. One-to-Many (PersistentUser.java - Parent):

问题 Somehow I can't figure out how to implement the following relations using Doctrine 2 syntax: I have Items and Shops. Each item has different price and different quantity at each shop. So, I have Items table, Shops table and ItemsAtShops table. How do I reflect the last one in Doctrine? I guess I need to create ItemsAtShops entity, relates ManyToOne -> Items and ManyToOne -> Shops, right? But in this case... how do I conveniently fetch a list of Items at the specific Shops with their prices

问题 I have a one to many relationship in my core data model. I need to create a new entity and save it. The entity has a one to many relationship which generated the following code: - (void)addRelationshipEvent1:(NSSet *)values; - (void)removeRelationshipEvent1:(NSSet *)values; . NSManagedObjectContext *context = [self.fetchedResultsController managedObjectContext]; NSEntityDescription *entity = [[self.fetchedResultsController fetchRequest] entity]; ApplicationRecord *newManagedObject =

问题 I have a social network set up and via an api I want to search the entries. The database of the social network is mysql. I want the search to return results in the following format: Results that match the query AND are friends of the user performing the search should be prioritized over results that simply match the query. So can this be done in one query or will I have to do two separate queries and merge the results and remove duplicates? I could possibly build up a data structure using

问题 I have two tables in my database, Operation and Equipment . An operation requires zero or more attributes. However, there's some logic in how the attributes are attributed: Operation Foo requires equipment A and B Operation Bar requires no equipment Operation Baz requires equipment B and either C or D Operation Quux requires equipment ( A or B ) and ( C or D ) What's the best way to represent this in SQL? I'm sure people have done this before, but I have no idea where to start. (FWIW, my

问题 I have three classes: public partial class Student : Contact { //Inherited from Contact: //public int ContactId { get; set; } //public string FirstName { get; set; } //public string LastName { get; set; } public virtual StudentExam StudentExam { get; set; } } public partial class Exam { public int ExamId { get; set; } public string Title { get; set; } public DateTime Date { get; set; } public virtual StudentExam StudentExam { get; set; } } public partial class StudentExam { public byte Score

问题 I am in the process of designing this E-R diagram for a shop of which I have shown part of below (the rest is not relevant). See the link please: E-R diagram The issue that I have is that the shop only sells two items, Socks and Shoes. Have I correctly detailed this in my diagram? I'm not sure if my cardinalities and/or my design is correct. A customer has to buy at least one of these items for the order to exist (but has the liberty to buy any number). The Shoe and Sock entities would have

问题 How to do json format using nltk. Example: tagged_sent = [('Rami', 'PERSON'), ('Eid', 'PERSON'), ('is', 'O'), ('studying', 'O'), ('at', 'O'), ('Stony', 'ORGANIZATION'), ('Brook', 'ORGANIZATION'), ('University', 'ORGANIZATION'), ('in', 'O'), ('NY', 'LOCATION')] I want output like json format: data = { 'Rami Eid':{'job': 'engineer', 'location':'NY'}, 'GuruRaj Bagali':{'job': 'professor', 'location': 'NY'} } 来源： https://stackoverflow.com/questions/35178585/how-to-do-json-format-using-nltk

问题 Database design question :) Is it smarter to make a lot of interconnected tables (normalize) or is it smarter to duplicate data so queries are simpler? Here is my situation: public class TransferRequest { [Key] public int TransferRequestId { get; set; } public int By { get; set; } public int? For { get; set; } public int PersonId { get; set; } public virtual Person Person { get; set; } [ForeignKey("Transfer")] public int? ExistingTransferId { get; set; } public virtual Transfer