entity-relationship

问题 When creating many to many relationships we use an intermediary table. Lets say I use the following entities video , category , tag , and VideoCategory , VideoTag to create the relations. I'm assuming that many tags/categories can have many videos and vice-versa. And I do it with through keyword 'cause I want to be able to use extra fields in the future if I want. class Category(models.Model): category = models.CharField(max_length=50) def __str__(self): return self.category class Tag(models

问题 For a better user experience, I didn't want to just make a casual PM system, but actually group it into conversations. And as an added feature, being able to PM multiple people at once. Something that was requested multiple times. So I created this: /* User conversations ONE Conversation HAS MANY Conversation Members HAS MANY Users HAS MANY Private Messages ONE Private Message HAS ONE Conversation ONE User HAS MANY Conversation Member HAS ONE Conversation */ CREATE TABLE IF NOT EXISTS `tbl

问题 Let say the data entities are: Bookshop <--->> Books How do I fetch all books belonging to a specific book shop that contain "cloud" in the name for example? The following method feel clunky. Bookshop *bookshop = (Bookshop *) nsManagedObjectFromOwner; NSString *searchTerm = @"cloud"; NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init]; NSEntityDescription *entity = [NSEntityDescription entityForName:@"Books" inManagedObjectContext:self.managedObjectContext]; [fetchRequest setEntity

问题 Dunno how to name this properly. I have two entities in m:n relationship: Member and Role. public class Role { public int Id { get; set; } public string Title { get; set; } public ICollection<Member> MembersInRole { get; set; } } public class Member { public int Id { get; set; } public string Name { get; set; } public string Password { get; set; } public string Email { get; set; } public ICollection<Role> Roles { get; set; } } I have made some seed data: http://i56.tinypic.com/2vjvj1w.png And

问题 I have two entities Entity1 and Entity2 with a OneToMany relation, but they live in two MySQL databases. How can I implement those entities with their relation in Symfony? Is it possible to create two separated bundles where to implement those entities? 回答1: In Doctrine, joining data across databases is not technically “supported” by a designed feature, but you can make it work by tricking Doctrine a little bit. If you want to build a relationship between entities then they must use the same

问题 Currently trying to get to grips with SQL for the first time, so I am working through a few problems. Here is a sample database spec: Students (name, gender, course) do projects(title). Each project has two supervisors (name, gender, department). All students do a project but not all projects get taken. More than one student can do the same project. Students meet one of their supervisors regular and these meetings are recorded (date, time, student, supervisor, notes). So far I've got an ER

问题 Does anyone know a way to make an ER diagram from SQLAlchemy models in python 3. I found sqlalchemy_schemadisplay, which is python 2 because of pydot and ERAlchemy which is also python 2 only. 回答1: You can try eralchemy. import matplotlib.pyplot as plt import matplotlib.image as mpimg import pandas as pd from eralchemy import render_er from sqlalchemy import (MetaData, Table, Column) metadata = MetaData() # create your own model .... users = Table('users', metadata, Column('user_id', Integer(

问题 What is the difference between ERD (Entity relationship diagram) and EERD (enhanced entity relationship diagram)? 回答1: Entity-Relationship model You have relationships (image source) and attributes (image source) that can also be primary keys (image source) Enhanced entity–relationship model You have the same as with the ER model plus specialisation partitioning (which are multiple specialisations) generalisation aggregation keys are annotated differently: 回答2: I would add that ER model is

问题 Hi I try use Many to Many relationship with EF Fluent API. I have 2 POCO classes. public class Project { public int ProjectId { get; set; } public virtual ICollection<Author> Authors { get; set; } public Project() { Authors = new List<Author>(); } } public class Author { public int AuthorId { get; set; } public virtual ICollection<Project> Projects { get; set; } public Author() { Projects = new List<Project>(); } } And I map many to many relationship with this part of code: ////MANY TO MANY

问题 iam new to mysqlworkbench. i am using the latest 6.1 version of it. While iam designing model(EER diagram) i created a table and tried to insert some default data to it in the bottom inserts tab of the respective table. now when i try to close the bottom tab i get a pop up for saying to apply the changes, but i cant find the apply button anywhere on the GUI , Please help where is the apply button so that i can apply 回答1: There's an apply button on the toolbar: 来源： https://stackoverflow.com