foreign-key-relationship

问题 I have a User < Country model. A user belongs to a country, but may not belong to any (null foreign key). How do I set this up? When I try to insert a user with a null country, it tells me that it cannot be null. The model is as follows: public class User{ public int CountryId { get; set; } public Country Country { get; set; } } public class Country{ public List<User> Users {get; set;} public int CountryId {get; set;} } Error: A foreign key value cannot be inserted because a corresponding

问题 I tried a bunch of different things first but don't seem to be getting a result. Controller: public function all() { $projects = Project::all(); foreach($projects as $project) { $pid = $project->id; } $am = DB::table('projects') ->join('employees', 'projects.am_id', '=', 'employees.id') ->where('projects.id', '=', $pid) ->select('projects.id', 'projects.am_id', 'employees.name') ->first(); $pm = DB::table('projects') ->join('employees', 'projects.pm_id', '=', 'employees.id') ->where('projects

问题 MESSAGE_MAP TABLE CREATE TABLE `message_map` ( `message_from` varchar(15) NOT NULL, `message_id` varchar(15) NOT NULL, `message_to` varchar(15) NOT NULL, `message_status` bit(1) DEFAULT NULL ) ENGINE=InnoDB DEFAULT CHARSET=latin1; ALTER TABLE `message_map` ADD PRIMARY KEY (`message_from`,`message_id`,`message_to`), ADD KEY `FK_ij6tystusydqijqp8lgoigo1c` (`message_id`), USER TABLE CREATE TABLE `user` ( `USER_ID` varchar(15) NOT NULL, `PASSWORD` varchar(15) DEFAULT NULL, `PHONE_NUMBER` varchar

问题 I want show related table columns ( customers.name ) in all select of model (User) laravel. I use accessor laravel. user table: id name customer_id 1 hassan 1 customer table: id name 1 customer1 now use $user = Auth::user(); return $user; I want show: id: 1, name: "hassan", customer_id: { id: 1, name: "customer1" } but show this error: Failed calling App\User::jsonSerialize() class User extends Authenticatable { use EntrustUserTrait; /** * Get the user's customer name. * * @param string

问题 I use MySQL with InnoDB engine. I double-checked type of columns. But always have: Error Code: 1215. Cannot add foreign key constraint I tried: ALTER TABLE `mail`.`boxes` ADD CONSTRAINT FK_id FOREIGN KEY (id) REFERENCES `mail`.`users` (id) ON UPDATE NO ACTION ON DELETE NO ACTION; and ALTER TABLE `mail`.`boxes` ADD FOREIGN KEY (id) REFERENCES `mail`.`users` (id) Nothing works((( Please, help, what I am doing wrong (except choosing MySQL :-) )? 回答1: If table contains data then you are not able

问题 i'm building a DB of a graduation projects management system. Students are divided into groups .There is groups table and faculty table. Each group has an advisor and two examiners. i'm confused here. Should i create 3 FKs from the the faculty table? 2 for examiners and 1 for advisor? here is the SQL code: create table groups ( groupID NUMBER not null, nbStudents NUMBER not null, avgGPA DOUBLE NOT NULL, projectName varchar(50) not null, advisorID NUMBER examiner1ID NUMBER examiner2ID NUMBER

问题 how to try like this, i already write in ItemsTable.php like this public function initialize(array $config) { $this->belongsTo('Categories', [ 'className' => 'Item', 'foreignKey' => 'cid', 'propertyName' => 'name' ]); } How do I display category name instead of Cid ? 来源： https://stackoverflow.com/questions/27718675/how-to-find-field-through-foreign-key-in-cakephp3-x-x

问题 I am trying to add a connection to a RestKit mapping but without success. My JSON is as follows: { parent: { token: "token" title: "title", description: "description", child_id: 1 } } My CoreData scheme defines that the Parent ManagedObject has a Child relation under the child key (and the opposite). The Parent mapping happens after I already have a corresponding Child object in CoreData, so that's not a problem. I followed some links and discussions around the web in order to solve this, but

问题 In my ASP.NET Core 1.1 , EF-Core 1.1 app , I'm displaying data in a parent view that has partial view . Scenario is similar to the following: one-to-many relationship between Customers and Orders table . User selects a customer from a dropdown in the Parent View that has a partial view that displays corresponding orders for the selected customer. But, I'm always getting model passed to the Partial view always null. Question : What I may be missing here? Note : By placing breakpoints, I've

问题 Is that possible to establish a relationship between Tree and Branch such as: class Tree < ActiveRecord::Base has_many :branches end class Branch < ActiveRecord::Base belongs_to :tree end But with an array of foreign keys branch_ids stored in Tree? I know it's the opposite of the default process, but I want to do so (just for testing). Many thanks for any help. 回答1: As Lichtamberg mentioned it is a bad schema. Since you said "just for testing", if branch ids will be a column with comma