many-to-many – Page 4

AttributeError: ‘ManyRelatedManager’ object has no attribute ‘add’? I do like in django website but got this error

June 14, 2023 by Tarik

JamesO is correct – it looks like your Category.articles field has a through relationship. Assuming that your models at least resemble the following class Article(models.Model): name = models.CharField(max_length=128) class Category(models.Model): name = models.CharField(max_length=128) articles = models.ManyToManyField(Article, through=”Membership”) class Membership(models.Model): article = models.ForeignKey(Article) category = models.ForeignKey(Category) author = models.CharField() then to add an Article to a … Read more

Laravel Eloquent ORM – Many to Many Delete Pivot Table Values left over

June 6, 2023 by Tarik

The detach method is used to release a relationship from the pivot table, whilst delete will delete the model record itself i.e. the record in the reviews table. My understanding is that delete won’t trigger the detach implicitly. You can use model events to trigger a cleanup of the pivot table, though, using something like: … Read more

Saving many-to-many relationship in Entity Framework Core

June 1, 2023 by Tarik

This is a simple and a very fundamental question to EF Core many-to-many relationship; I do not know why no one has written a complete example for n..m in EF Core. I have modified your code (primary key as int), I do not like string in primary key. Just copy/paste the code and every should … Read more

Self-referencing many-to-many recursive relationship code first Entity Framework

May 26, 2023 by Tarik

By convention, Code First will take uni-directional associations as one to many. Therefore you need to use fluent API to let Code First know that you want to have a many to many self referencing association: protected override void OnModelCreating(ModelBuilder modelBuilder) { modelBuilder.Entity<Member>().HasMany(m => m.Friends).WithMany().Map(m => { m.MapLeftKey(“MemberId”); m.MapRightKey(“FriendId”); m.ToTable(“MembersFriends”); } ); }

How to sort NULL values last using Eloquent in Laravel

May 24, 2023 by Tarik

Laravel does not take into consideration the ISNULL method however, you can pass it in as a raw query and still make use of it as it’s more efficient than IF statements and the results will stay the same if you ever go beyond 1000000 employees (accepted answer), like so: public function employees() { return … Read more

Get objects from a many to many field

May 20, 2023 by Tarik

You can use: space.related.all() to return a QuerySet of User

Many-to-many relationships examples

May 19, 2023 by Tarik

Example scenario: students and courses at a university. A given student might be on several courses, and naturally a course will usually have many students. Example tables, simple design: CREATE TABLE `Student` ( `StudentID` INT UNSIGNED NOT NULL AUTO_INCREMENT, `FirstName` VARCHAR(25), `LastName` VARCHAR(25) NOT NULL, PRIMARY KEY (`StudentID`) ) ENGINE=INNODB CHARACTER SET utf8 COLLATE utf8_general_ci … Read more

How do I access the properties of a many-to-many “through” table from a django template?

May 18, 2023 by Tarik

The easiest way is just to pass the band to the template. Templates are capable of navigating the relationships between models and there is both members and membership_set queryset managers on Group. So here is how I would do it: view: def group_details(request, group_id): group = get_object_or_404(Group, pk=group_id) return render_to_response(‘group_details.html’, {‘group’: group}) template: <h2>{{ group.name … Read more

SQL JOIN many-to-many

May 18, 2023 by Tarik

It’s possible with this little trick (OUTER JOIN on the many-to-many table, with the constraint that the GroupID has to be 3 (for Drama) http://sqlfiddle.com/#!9/01cf3/1 SELECT elements.ID, elements.Element, groups.Genre FROM elements LEFT OUTER JOIN group_elements ON elements.ID = group_elements.ElementID AND group_elements.GroupID = 3 LEFT OUTER JOIN groups ON group_elements.GroupID = groups.ID LEFT OUTER JOIN means … Read more

Query a many-to-many relationship with linq/Entity Framework. CodeFirst

May 16, 2023 by Tarik

You can do this: var cat_id=1; // Change this variable for your real cat_id var query= from article in db.Articles where article.Categories.Any(c=>c.Category_ID==cat_id) select article; This way you will get the articles that satisfies the condition you want. This is the sql code that is generated by that query: SELECT [Extent1].[Id] AS [Id], [Extent1].[Title] AS [Title] … Read more