entity-relationship

问题 I am designing a database that handles users, accounts and projects with the following relationships and constraints: An account has many users A user belongs to many accounts An account has many projects A project belongs to only one account A user collaborates in many projects (redundant note: each one of them belonging to its own account). In other words, a user can collaborate in many projects of the same account. But since a user can belong to several accounts, thus a user can

问题 I'm using EF and when I do this: foreach (var reg in detail.Regs) { this.db.Regs.DeleteObject(reg); } I get this: Collection was modified; enumeration operation may not execute. What I'm I doing wrong here??? 回答1: The reason for that is because as you delete the objects from the context, EF is actively update the Regs navigation property count which means the detail.Regs collection is being changed during the foreach loop which always cause the exception you are getting. You can create a new

问题 I have a class of Content which should be able to have a parentId for inheritance but also I want it to have a list of child content which is nothing to do with this inheritance tree. I basically wanted a link table as ChildContentRelationship with Id's for parentContent and childContent in it and the Content class would have a list of ChildContentRelationship. This has caused a lot of errors. Here's waht I sort of want to do public class Content { public int Id { get; set; } public int?

问题 Hi i have that same question as here: Many-to-many self relation with extra fields? but i cant find an answer :/ I tried first ManyToOne and at the other site OneToMany ... but then i could not use something like public function hasFriend(User $user) { return $this->myFriends->contains($user); } because there was some this problem: This function is called, taking a User type $user variable and you then use the contains() function on $this->myFriends. $this->myFriends is an ArrayCollection of

问题 I have a custom users table in my database and i want to create a one-to-one relationship with aspnetusers table so that when i register a new customer, applicationuser class through UserManager should add the username, email, password and school code to the users table and add an fk in its own table. IS there any tutorial/example implementing this kind of scenario? I am trying to add a one-to-one relationship with my users table but the fk is different than pk Users Table [columns] username

问题 Say I run an online business where you can order products from my website, and I have a database with two tables: Table order with the fields order_number, customer_ID, address Table customer with the fields customer_ID, first_name, last_name To get a full, detailed 'report' of the order, I would perform a LEFT JOIN to concatenate data from the order table to include the customer's first and last names along with their address and order number. My question is, how are these tables related?

问题 So, I've read a whole lot of answers here on stackoverflow, but I'm still confused about the whole concept thereof. Specifically, I've gone over this article (including all the ones it references), but can't seem to find a solid grasp on the concept (or perhaps it is my confusion between cardinality (n:m, etc.) and identities): Still Confused About Identifying vs. Non-Identifying Relationships My issue is this: I know that identifying relationships imply that the primary key of a child entity

问题 I'm trying to create a quick ASP.NET MVC 3 application using the RC version of EF 4.1. I have two models: public class Race { public int RaceId { get; set; } public string RaceName { get; set; } public string RaceDescription { get; set; } public DateTime? RaceDate { get; set; } public decimal? Budget { get; set; } public Guid? UserId { get; set; } public int? AddressId { get; set; } public virtual Address Address { get; set; } } and public class Address { public int AddressId { get; set; }

问题 I am currently setting up a database which has a large number of many-to-many relations. Every relationship was modeled via a link table. Example: A person has a number of jobs, jobs are fulfilled by a number of persons. A person has a number of houses, houses are occupied by a number of persons. A person has a number of restaurants he likes, restaurants have a number of persons who like the restaurant. First I designed this as follows: Tables: Person, Job, House, Restaurant, Person_Job,

问题 Is there any way to get an ER model of a database from the server that is connected to my MySQL Workbench? 回答1: Go to "Database" Menu option Select the "Reverse Engineer" option. A wizard will be open and it will generate the ER Diagram for you. 回答2: I want to enhance Mr. Kamran Ali's answer with pictorial view. Pictorial View is given step by step: Go to " Database " Menu option Select the " Reverse Engineer " option. A wizard will come. Select from " Stored Connection " and press " Next "