I have 3 tables:
Table A
Table B
Table C
Table A has a foreign key to Table B. Table C has a foreign key to Table B. So B acts as an intermediary through table, but it does not have any foreign keys to A or C itself, so I cannot declare a ManyToManyField from A to C in the conventional way, but I would like to do so in order to use prefetch_related when grabbing C records from A. Is there a way to do this?
Related
I'm having a problem and don't have much time so solve it.
I have thoses tables in my database, one called wall and the other is users.
For the wall table, I can add a foreign like you can see :
But in the users table, I can't and I don't know why. Both tables are empty.
Here you can see the users table :
And this is the error code I get :
I think there is not possible to create foreign key to the same table.
In this SQL you change table 'user' and add to this table foreign key from 'user' table.
If I'm wrong please correct me.
Here is example:
alter table `wall` add constraint `id_user_constraint_name` foreign key (`id_user`) references `users`(id);
If you don't have a 'id_user' column use this sql:
alter table `wall` add column id_user bigint;
I'm trying to add unique columns on a pivot table created via a ManyToMany association.
I found this page of the documentation explaining how to generate a database unique constraint on some columns with this example:
/**
* #Entity
* #Table(name="ecommerce_products",uniqueConstraints={#UniqueConstraint(name="search_idx", columns={"name", "email"})})
*/
class ECommerceProduct
{
}
But this only works if I create the pivot table via a third entity and, in my case, I created the pivot table using a ManyToMany relation (in the same fashion as this code).
Is there a way to add unique columns on pivot table while still using ManyToMany or do I need to rely on a third entity?
While #Table annotation proposes a uniqueConstraints option, #JoinTable does not. Thus, if you want to add a unique constraint on your association table, you will have to actually create another entity explicitly.
That being said, the default join table should not need anything more than the default configuration set up by Doctrine. Currently, when adding a ManyToMany association, the join table is composed of two fields and a composite primary key relying on both fields is created.
If your association table only contains the two basic fields referring to both sides of your association (which is necessarily the case if you use #ManyToMany), the composite primary key should be all you need.
Here is the generated SQL for the basic example where a User has a ManyToMany association with Group (from this section of the documentation):
CREATE TABLE users_groups (
user_id INT NOT NULL,
group_id INT NOT NULL,
PRIMARY KEY(user_id, group_id)
) ENGINE = InnoDB;
ALTER TABLE users_groups ADD FOREIGN KEY (user_id) REFERENCES User(id);
ALTER TABLE users_groups ADD FOREIGN KEY (group_id) REFERENCES Group(id);
As you can see, everything is properly set up with a composite primary key which will ensure that there can't be duplicate entries for the couple (user_id, group_id).
Of course there is another alternative, Alan!
If you need a Zero to Zero relationship, the only alternative is defining the unique constraint per each pk in the agregated table, to make doctrine figuring out about zero to zero relationship.
The problem is that Doctrine's people hadn't considered zero to zero relationships, so the only alternative for this is manytomany relationship with one unique constraint per pk.
If you have doubts about final-state of your doctrine implementation of your E-R model, I strongly recommend mysql-workbench-schema-exporter. With this php tool, you can easily export your mysql workbench E-R schema to a Doctrine's working classes schema, so you would be able to easily explore all your alternatives ;-)
Hope this helps
I have a table T1 with 4 fields (F1, F2, F3 and F4) together acting as unique identifier of the table rows. Can one of these fields (F4 say) be used as a foreign key for the same table? We already have master data table (T2) for the same.
Yes, it can.
You can easily set one of the primary key fields as a foreign key and assign it a check table. By default SAP proposes you domain value table of this field as a check table for it.
Moreover, while creating you can specify the role of a foreign key field in your T1 table: either it is key or non-key.
The only limitation: you cannot create foreign key for a field if it's already assigned the same value table in its domain as current. For example, one cannot make field MATNR in MARA foreign key, because it has value table MARA in its domain. You'll get an error E2165:
Check table of the domain is current table (FK maintenance not possible)
I am drawing up an ER model for a to-be application using MySQL workbench.
Being new to MySQL workbench, just wanted to understand how creating a foreign-key and 1-to-1 identifying relationship are different
Table: Employee
Column1: EmployeeID, Type INT [Primary Key]
Column2: Name, Type Varchar(45)
Table: EmployeeDetails
Column1: EmployeeID, Type INT [Primary Key]
Column2: Address, Varchar(100)
If I create a foreign key on EmployeeDetails table employeeid column that references employee table EmployeeID column, how is it different than creating a 1-to-1 identifying relationship?
Foreign keys and relationships are the same concept, just from different views. FKs are the entitity used in the database while relationships are their visualization in a model. In your model you only create relationships. You can edit their properties in the relationship editor (e.g. by double clicking on a relationship) to be 1:1, 1:n, identifying or not, mandatory etc. The different relationship tools are just convenience tools for the same thing.
Good morning. I have been looking all over trying to answer this question.
If you have a table that has foreign keys to another table, and you want results from both tables, using basic sql you would do an inner join on the foreign key and you would get all the resulting information that you requested. When you generate your JPA entities on your foreign keys you get a #oneToone annotation, #oneToMany, #ManyToMany, #ManyToOne, etc over your foreign key columns. I have #oneToMany over the foreign keys and a corresponding #ManyToOne over the primary key in the related table column I also have a #joinedON annotation over the correct column... I also have a basic named query that will select everything from the first table. Will I need to do a join to get the information from both tables like I would need to do in basic sql? Or will the fact that I have those annotations pull those records back for me? To be clear if I have table A which is related to Table B based on a foreign key relationship and I want the records from both tables I would join table A to B based on the foreign key or
Select * From A inner Join B on A.column2 = B.column1
Or other some-such non-sense (Pardon my sql if it is not exactly correct, but you get the idea)...
That query would have selected all column froms A and B where those two selected column...
Here is my named query that I am using....
#NamedQuery(name="getQuickLaunch", query = "SELECT q FROM QuickLaunch q")
This is how I am calling that in my stateless session bean...
try
{
System.out.println("testing 1..2..3");
listQL = emf.createNamedQuery("getQuickLaunch").getResultList();
System.out.println("What is the size of this list: number "+listQL.size());
qLaunchArr = listQL.toArray(new QuickLaunch[listQL.size()]);
}
Now that call returns all the columns of table A, but it lack's the column's of table B. My first instinct would be to change the query to join the two tables... But that kind of makes me think what is the point of using JPA then if I am just writing the same queries that I would be writing anyway, just in a different place. Plus, I don't want to overlook something simple. So what say you stack overflow enthusiasts? How does one get back all the data of joined query using JPA?
Suppose you have a Person entity with a OneToMany association to the Contact entity.
When you get a Person from the entityManager, calling any method on its collection of contacts will lazily load the list of contacts of that person:
person.getContacts().size();
// triggers a query select * from contact c where c.personId = ?
If you want to use a single query to load a person and all its contacts, you need a fetch in the SQL query:
select p from Person p
left join fetch p.contacts
where ...
You can also mark the association itself as eager-loaded, using #OneToMany(lazy = false), but then every time a person is loaded (vie em.find() or any query), its contacts will also be loaded.