many-to-many

问题 I'm working on building a database to manage project assignments, and the one part that's giving me particular trouble is the assignment of job titles to employees with for each project they are working on. Requirements An Employee can be on multiple Projects at a time A Project has multiple Employees on it A Project has multiple Job Titles An Employee works on a Project under exactly one of the Project's Job Titles Multiple Employees can work under the same Job Title in a Project I'm not

问题 I have a site like SO, Wordpress, etc, where you make a post and u can have (optional) tags against it. What is a common database schema to handle this? I'm assuming it's a many<->many structure, with three tables. Anyone have any ideas? 回答1: A three table many to many structure should be fine. Eg. Posts, PostsToTags(post_id,tag_id), Tags The key is indexing. Make sure you PostsToTags table is indexed both ways ( post_id,tag_id and tag_id,post_id ) also if read performance is ultra critical

问题 I am a new rails user. This is probably a very basic question, but I need help I am making an app that has a Bands table, a Festivals table, and a Participations table in a many-to many relationship. My models are as follows: class Band < ActiveRecord::Base attr_accessible :year, :genere, :name, :country has_many :participations has_many :festivals, :though => :participations end class Festival < ActiveRecord::Base attr_accessible :city, :name, :country has_many :participations has_many

问题 I have three tables in the many-to-many format. I.e, table A, B, and AB set up as you'd expect. Given some set of A ids, I need to select only the rows in AB that match all of the ids. Something like the following won't work: "SELECT * FROM AB WHERE A_id = 1 AND A_id = 2 AND A_id = 3 AND ... " As no single row will have more than one A_id Using, an OR in the sql statment is no better as it yields results all results that have at least one of the A ids (whereas I only want those rows that have

问题 This is an update to my previous question : Many to many relationship The previous solution works fine, but now I want tgo improve the results a little bit. I´d like to have all the wavelength values in one row. So instead of the following result : DateTimeID Wavelength SensorID 11435 1581,665 334 11435 1515,166 334 11435 1518,286 335 I'd like to have something similar to this: DateTimeID Wavelength1 Wavelength2 SensorID 11435 1581,665 1515,166 334 11435 1518,286 335 回答1: You could use the

问题 I am having an issue with many-to-many mapping using NHibernate. Basically I have 2 classes in my object model (Scenario and Skill) mapping to three tables in my database (Scenario, Skill and ScenarioSkill). The ScenarioSkills table just holds the IDs of the SKill and Scenario table (SkillID, ScenarioID). In the object model a Scenario has a couple of general properties and a list of associated skills (IList) that is obtained from the ScenarioSkills table. There is no associated IList of

问题 There are 3 tables: shelf, section and shelf_has_section intermediate table to support n-m relation. The schema build from symfony doctrine:build-schema looks as following. Simply, shelf(id, position) section(id, name) shelf_has_section(shelf_id, section_id, number_of_books) The schema. Shelf: connection: doctrine tableName: shelf columns: id: type: integer(4) fixed: false unsigned: true primary: true autoincrement: true position: type: string(255) primary: false notnull: true autoincrement:

问题 I build an app in django, but since I found out that google app engine doesn't support Django out of the box (free,cloud sql can't be used for free right?). I decided to move to Django-nonrel, so there are few datebase Field that need converting, and I don't know how: class Cate(models.Model): name = models.CharField(max_length = 100) description = models.TextField() create_by = models.ForeignKey(User) create_date = models.DateTimeField('cate created date') def __unicode__(self): return self

问题 I found this answer which (unless I'm missing something) gives a good solution to catch all changes made to all independent associations in a DbContext. I am trying to adapt that solution to something more specific, to create a function that can find the changes for a particular navigation property on a particular entity. The goal is for the user of this function to not have to worry about any of the internals of EntityFramework, and to only be concerned with his POCO models. This is what I

问题 class User(models.Model): name = models.CharField(max_length=100) age = models.IntegerField() gender = models.IntegerField() email = models.CharField(max_length=100) password = models.CharField(max_length=255) following = models.ManyToManyField("self", related_name='followers') objects = UserManager() def __repr__(self): return "User: {0}".format(self.name) In my model, User, users can follow and followed by each other. I can find who the user is following by this: user1 = User.objects.get(id