Questions tagged [many-to-many]
A type of relationship between entities of types A and B which associates a list of entities of type B to an entity of type A and vice versa. Types A and B may be the same type.
8,780
questions
677
votes
6
answers
443k
views
What is related_name used for?
What is the related_name argument useful for on ManyToManyField and ForeignKey fields? For example, given the following code, what is the effect of related_name='maps'?
class Map(db.Model):
...
- 65.1k
329
votes
7
answers
154k
views
Create code first, many to many, with additional fields in association table
I have this scenario:
public class Member
{
public int MemberID { get; set; }
public string FirstName { get; set; }
public string LastName { get; set; }
public virtual ICollection&...
- 3,293
310
votes
2
answers
129k
views
How do I make many-to-many field optional in Django?
When you have a many-to-many relationship (related_name, not through) and you are trying to use the admin interface you are required to enter one of the relationships even though it does not have to ...
- 3,221
223
votes
8
answers
214k
views
Difference Between One-to-Many, Many-to-One and Many-to-Many?
Ok so this is probably a trivial question but I'm having trouble visualizing and understanding the differences and when to use each. I'm also a little unclear as to how concepts like uni-directional ...
- 12.6k
210
votes
5
answers
119k
views
Rails find_or_create_by more than one attribute?
There is a handy dynamic attribute in active-record called find_or_create_by:
Model.find_or_create_by_<attribute>(:<attribute> => "")
But what if I need to find_or_create by more than ...
- 49.1k
194
votes
5
answers
273k
views
In which case do you use the JPA @JoinTable annotation?
In which case do you use the JPA @JoinTable annotation?
- 2,953
187
votes
4
answers
170k
views
Django removing object from ManyToMany relationship
How would I delete an object from a Many-to-Many relationship without removing the actual object?
Example:
I have the models Moods and Interest.
Mood has a many-to-many field interests (which is a ...
- 15.5k
164
votes
5
answers
102k
views
MongoDB Many-to-Many Association
How would you do a many-to-many association with MongoDB?
For example; let's say you have a Users table and a Roles table. Users have many roles, and roles have many users. In SQL land you would ...
- 23.1k
160
votes
1
answer
162k
views
How to implement a many-to-many relationship in PostgreSQL?
I believe the title is self-explanatory. How do you create the table structure in PostgreSQL to make a many-to-many relationship.
My example:
Product(name, price);
Bill(name, date, Products);
- 20.3k
156
votes
5
answers
73k
views
SQL - many-to-many table primary key
This question comes up after reading a comment in this question:
Database Design
When you create a many-to-many table, should you create a composite primary key on the two foreign key columns, or ...
- 87.4k
143
votes
3
answers
132k
views
Mapping many-to-many association table with extra column(s)
My database contains 3 tables:
User and Service entities have many-to-many relationship and are joined with the SERVICE_USER table as follows:
USERS - SERVICE_USER - SERVICES
SERVICE_USER table ...
- 1,613
117
votes
5
answers
148k
views
Insert/Update Many to Many Entity Framework . How do I do it?
I'm using EF4 and new to it. I have a many to many in my project and cannot seem to work out how to insert or update. I have build a small project just to see how it should be coded.
Suppose I have 3 ...
- 15.8k
113
votes
4
answers
114k
views
Django filter many-to-many with contains
I am trying to filter a bunch of objects through a many-to-many relation. Because the trigger_roles field may contain multiple entries I tried the contains filter. But as that is designed to be used ...
- 1,244
104
votes
2
answers
42k
views
Whats the difference between a OneToOne, ManyToMany, and a ForeignKey Field in Django?
I'm having a little difficulty getting my head around relationships in Django models.
Could someone explain what the difference is between a OneToOne, ManyToMany and ForeignKey?
- 1,195
97
votes
1
answer
56k
views
Django's ManyToMany Relationship with Additional Fields
I want to store some additional information in that, automatically created, ManyToMany join-table. How would I do that in Django?
In my case I have two tables: "Employees" and "Projects". What I ...
- 3,026
86
votes
3
answers
129k
views
getting the value of an extra pivot table column laravel
I have a phone_models, phone_problems, and a phone_model_phone_problem pivot table. The pivot table has an extra column 'price'.
PhoneModel:
class PhoneModel extends \Eloquent
{
public function ...
- 3,199
86
votes
7
answers
37k
views
How do I access the child classes of an object in django without knowing the name of the child class?
In Django, when you have a parent class and multiple child classes that inherit from it you would normally access a child through parentclass.childclass1_set or parentclass.childclass2_set, but what ...
- 39.9k
84
votes
2
answers
96k
views
What is the difference between fetch="EAGER" and fetch="LAZY" in doctrine
What is the difference between fetch="EAGER" and fetch="LAZY" in annotation @ManyToOne in Doctrine ?
/**
* @ManyToOne(targetEntity="Cart", cascade={"all"}, fetch="EAGER")
*/
/**
* @ManyToOne(...
- 2,736
81
votes
6
answers
46k
views
Django Many-to-Many (m2m) Relation to same model
I'd like to create a many-to-many relationship from and to a user class object.
I have something like this:
class MyUser(models.Model):
...
blocked_users = models.ManyToManyField(MyUser, ...
- 22.9k
68
votes
4
answers
36k
views
UnsupportedOperationException merge-saving many-to-many relation with hibernate and JPA
I've set up a simple many-to-many relationship account : role with Hibernate but when I try to save an account in a unit test after it has had its role added I get an UnsupportedOperationException:
...
- 10.8k
67
votes
4
answers
83k
views
JPA Hibernate many-to-many cascading
I am using JPA 2.0 and hibernate. I have a User class and a Group class as follows:
public class User implements Serializable {
@Id
@Column(name="USER_ID")
private String userId;
@...
- 7,559
66
votes
3
answers
32k
views
How to organise a many to many relationship in MongoDB
I have two tables/collections; Users and Groups. A user can be a member of any number of groups and a user can also be an owner of any number of groups. In a relational database I'd probably have a ...
- 6,014
62
votes
2
answers
55k
views
Symfony2-Doctrine: ManyToMany relation is not saved to database
I have two PHP model classes named Category and Item. A Category may have many Items and an Item may belong to many Categories.
I have created a ManyToMany relation to both classes:
class Category
{
...
- 5,808
61
votes
1
answer
65k
views
Many-to-many mapping table
From examples that I have seen online and in a Programming Entity Framework CodeFirst book, when you have a collection on both classes EF would create a mapping table such as MembersRecipes and the ...
- 5,859
59
votes
2
answers
29k
views
SQLAlchemy ManyToMany secondary table with additional fields
I have 3 tables: User, Community, community_members (for relationship many2many of users and community).
I create this tables using Flask-SQLAlchemy:
community_members = db.Table('community_members'...
- 1,575
57
votes
2
answers
82k
views
How to create a many-to-many mapping in Entity Framework?
Here is the case, I have 2 entities, such as Contract、Media。
public class Media : Entity
{
public string Name {get; set;}
public bool Enabled
*//other properties can be ignored..*
}
...
- 649
57
votes
1
answer
17k
views
How to query directly the table created by Django for a ManyToMany relation?
I have a model MyModel2 with a ManyToManyField related to another model MyModel1.
How can I get the pairs mymodel1.id, mymodel2.id, as represented in the table Django creates for this relation? Do I ...
- 36.9k
55
votes
2
answers
49k
views
listing objects from ManyToManyField
I am trying to print a list of all the Conferences and for each conference, print its 3 Speakers.
In my template I have:
{% if conferences %}
<ul>
{% for conference in ...
- 1,015
52
votes
4
answers
29k
views
How to add column in ManyToMany Table (Django)
From the example of Django Book, I understand if I create models as following:
from xxx import B
class A(models.Model):
b = ManyToManyField(B)
The Django would create a new table(A_B) beyond ...
- 759
52
votes
3
answers
59k
views
How to build many-to-many relations using SQLAlchemy: a good example
I have read the SQLAlchemy documentation and tutorial about building many-to-many relation but I could not figure out how to do it properly when the association table contains more than the 2 foreign ...
- 10.3k
52
votes
3
answers
21k
views
Generic many-to-many relationships
I'm trying to create a messaging system where a message's sender and recipients can be generic entities. This seems fine for the sender, where there is only object to reference (GenericForeignKey) but ...
- 8,328
51
votes
4
answers
41k
views
Get all table names in a Django app
How to get all table names in a Django app?
I use the following code but it doesn't get the tables created by ManyToManyField
from django.db.models import get_app, get_models
app = get_app(app_name)
...
- 17.6k
51
votes
1
answer
143k
views
SQL JOIN many-to-many
Sorry about the minimalistic title but I don't know how to describe it in short. I have three tables:
The table of groups
ID | Genre
-----------------
1 | Action
2 | Adventure
3 | Drama
Many to ...
- 617
50
votes
5
answers
77k
views
Many-to-many relationships examples
I haven't found any MYSQL many-to-many relationships examples here and in google. What I am looking is to see a very simple example with php+mysql showing database's results. Can anybody write a very ...
- 21.4k
50
votes
6
answers
58k
views
Query a many-to-many relationship with linq/Entity Framework. CodeFirst
How can I query a many-to-many relationship using Entity Framework code first and linq? The problem is that EF create automatically the relation table. So, I don't have it in my context.
This is the ...
- 4,169
50
votes
3
answers
36k
views
How do I access the properties of a many-to-many "through" table from a django template?
From the Django documentation...
When you're only dealing with simple many-to-many relationships such as mixing and matching pizzas and toppings, a standard ManyToManyField is all you need. However, ...
- 2,380
49
votes
12
answers
53k
views
How to sort NULL values last using Eloquent in Laravel
I've got a many-to-many relationship between my employees and the group table. I've created the pivot table, and all is working correctly with that. However, I've got a sortOrder column on my ...
- 517
48
votes
3
answers
89k
views
Get objects from a many to many field
I have an m2m field called "admins" inside a model and I need to obtain from a view all the selected entries in that field, which are user IDs. Then with the user IDs obtain the emails for ...
- 8,314
46
votes
6
answers
79k
views
Laravel Eloquent ORM - Many to Many Delete Pivot Table Values left over
Using Laravel, I have the following code
$review = Review::find(1);
$review->delete();
Review has a many to many relationship defined with a Product entity. When I delete a review, I'd expect it ...
- 1,065
46
votes
5
answers
20k
views
Self-referencing many-to-many recursive relationship code first Entity Framework
I can't seem to make this work at all
class Member
{
public virtual IList<Member> Friends { get; set; }
[Key]
public int MemberId { get; set; }
public string Name{ get; set; }
}
...
- 12.3k
45
votes
1
answer
40k
views
Saving many-to-many relationship in Entity Framework Core
For example, I have 3 classes, which I'm using for many-to-many relationship:
public class Library
{
[Key]
public string LibraryId { get; set; }
public List<Library2Book> ...
- 5,573
44
votes
3
answers
21k
views
How to properly index a linking table for many-to-many connection in MySQL?
Lets say I have a simple many-to-many table between tables "table1" and "table2" that consists from two int fields: "table1-id" and "table2-id". How should I index this linking table?
I used to just ...
- 110k
43
votes
1
answer
40k
views
AttributeError: 'ManyRelatedManager' object has no attribute 'add'? I do like in django website but got this error
for item in data:
category_id = item['category_id']
del item['category_id']
category = Category.objects.get(pk=category_id)
code = item['code']
try:
article = Article....
- 15.7k
43
votes
1
answer
18k
views
Flask/SQLAlchemy - Difference between association model and association table for many-to-many relationship?
I started learning this stuff from the Flask Mega Tutorial. When he gets into Many-to-Many relationships, he creates an association table like this:
followers = db.Table('followers',
db.Column('...
- 4,025
42
votes
2
answers
34k
views
@ManyToMany(mappedBy = "foo")
Foo has:
@ManyToMany(mappedBy = "foos")
private Set<Bar> bars
and Bar has :
@ManyToMany
private Set<Foo> foos
What difference does the location of mappedBy attribute make to a bi-...
- 46.9k
42
votes
1
answer
5k
views
Search Core Data for all objects with an empty "to-many" relationship
In an almost identical situation to Core Data - Fetch all objects NOT in a relationship, only I'm looking for all records of one type that are not in any to-many relationship with another type.
So let'...
- 6,364
41
votes
3
answers
44k
views
how to verify if object exist in manytomany
Models
class Beer(models.Model):
pass
class Salas(models.Model):
name = models.CharField(max_length=20)
beers = models.ManyToManyField('Beer', blank=True)
View
beer = Beer.objects....
- 427
41
votes
1
answer
53k
views
How do I work with many-to-many relations in Yii2
For example in one-to-many due to documentation (http://www.yiiframework.com/doc-2.0/guide-db-active-record.html#relational-data) you can link two models in this way (one-many = company-zone):
$...
- 1,499
40
votes
3
answers
98k
views
MS SQL creating many-to-many relation with a junction table
I'm using Microsoft SQL Server Management Studio and while creating a junction table should I create an ID column for the junction table, if so should I also make it the primary key and identity ...
- 804
40
votes
20
answers
19k
views
Java method naming conventions: Too many getters
Why do Java method names use the "get" prefix so extensively? At least in my Java programs there are a lot of methods with names starting with the word "get". The percentage of get-methods is ...
- 2,098