I have retrieved an entity from the database. I would like to update one field (the modified date field) then insert it back to the database as a new entity. How would I achieve that?
Related
Is it possible to change primary key of many to many field from default to uuid?
Table is already populated. What is the best way for migration?
You can create a migration that executes raw queries, add a new field to the table in the middle then generate the new UUID.
After that, another set of queries to drop the constraints on ID, add the new constraints to the new UUID field, and lastly drop the old ID field.
https://docs.djangoproject.com/en/2.0/ref/migration-operations/#runsql
I have three tables "category", "product" and "product_category".
First two tables are created from two entity Category and Product.
Third table "product_category" is auto generated by framework doctrine console command.
Now I can get (fetch) product relational data (based on category id) from below query, which is fine.
$this->createQueryBuilder('p')
->leftJoin('p.category', 'c')
->select('p')
->where('c.id = :category_id')
->setParameter('category_id', 2)
->getQuery()->getSQL();
But how can I use Many-to-Many relation to Update Data?
I had Tried with several queries but its not working!!!
( I want to update all product status to inactive (2), whose category status is (2 = Inactive).
When doing a query on a table that has children. You can also just fetch the parent item, and doctrine will automatically handle the child objects. If you have a getCategory within the Product entity, it will resolve the category automatically.
Probably what you are looking for is cascading of events. of which is explained within Doctrine 2 ManyToMany cascade
I have to table with relation.
State
id
name
City
id
name
state
Which is better in performance?
city.state.id or city.state_id
city.state_id is better anyway. city.state will do another fetch from database.You can avoid this using select_related.If you need only id of foriegn key, no need of select_related here.Just do city.state_id(since foriegn key id will fetch in the query which gives city object).
city.state_id is better than city.state.id. Because It makes only a query instead of two.
BTW, You can use Django Debug Toolbar for debugging queries.
the <field>_id field you see is the database column name
docs
Behind the scenes, Django appends "_id" to the field name to create its database column name. In the above example, the database table for the Car model will have a manufacturer_id column
So this means it doesn't need to make a separate query to retrieve the foreign key instance (See Select a single field from a foreign key for more details).
But this assumes you haven't used select_related or prefetch_related
I am trying to work with QSqlRelationalTableModel of QT. I am new to MySQL table relationship but still I tried and can't make it work properly in QT.
I can get the result from MySQL:
create table stu(idd int auto_increment primary key,stu_name varchar(60),stu_age int);
create table stuInfo(idd int auto_increment primary key,stu_city varchar(60),stu_sub varchar(100), foreign key(id) references stu(id));
select stu.stu_name,stuInfo.stu_city from stu inner join stuInfo on stu.id=stuInfo.id;
To retrieve data from MySQL :
select stu.stu_name,stuInfo.stu_city from stu inner join stuInfo on stu.id=stuInfo.id;
In QT I can't make it work. I am getting confused with setRelation() and QSqlRelation() . I am not exactly understanding that how I can execute the same query in QT, I tried it in various way but sometime I get blank data, ugly header, errors etc.
Here is my learning code:
model = new QSqlRelationalTableModel();
model->setTable("stu");
model->setRelation(0,QSqlRelation("stu","id","stu_name","stu_age"));
model->setRelation(0,QSqlRelation("stuInfo","id","stu_city","stu_sub"));
model->select();
ui->tableView->setModel(model);
A QSqlRelation replaces the value of a field by the value of the other field in the relation, the replaced field won't appear in the query anymore, so you can't have 2 relations assigned to the same column, and you can't assign a relation to the primary key (as stated in the documentation of setRelation).
Basically the structure for which QSqlRelationalTableModel should be used would be a main table which would have 1 or more foreign index fields, and each of these fields could be replaced by the value of a chosen field in the tables from which the foreign indexes comes from (e.g.: to replace a "city_id" numerical field in the main table by the name of the city coming from another table for which that "city_id" is the primary key).
For what you want to do, you should use QSqlQueryModel with a manually constructed query instead of QSqlRelationalTableModel.
The problem is that your code does not really express the model you described.
You have a primary table called stuInfo, which references another table called stu.
To do this in Qt, you should create a table based on "stuInfo" (and not "stu"!):
model=new QSqlRelationalTableModel();
model->setTable("stuInfo");
Then you can implement your foreign key, as a relation:
model->setRelation(3,QSqlRelation("stu","id","stu_name"));
You need to point to index "3", which is the position of the reference field "id", on stuInfo table (0 will point to the primary key, which is not what you want!). The parameters of the QsqlRelation are the reference table name ("stu") the primary field name ("id") and the reference table field to which you want to point: in this case I am pointing to "stu_name"; if I wanted to point to the age, I could do something like this instead:
model->setRelation(3,QSqlRelation("stu","id","stu_age"));
After this code:
model->select();
ui->tableView->setModel(model);
you should have a view that shows you all the fields on stuInfo, and whose last field ("id") is mapped to the name (or age) on the "stu" table;
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.