I have the following query:
$qb = $this->em->createQueryBuilder()
->select(array('p','c' ,'sc'))
->from('Project\Entity\Product', 'p')
->innerJoin("p.category", "c")
->innerJoin("p.subcategory", "sc")
->where("p.available != 0")
->orderBy("p.create_date")
->addOrderBy("p.id")
->setMaxResults($limit);
In a result I would like to get only Products with id's of related entities (category and subcategory).
How can I achieve that?
I'm afraid you can't do that.
Either you use fetch joins and obtain fully loaded objects or load just the object and obtain an uninitialized collection (which requires one more query to be initialized).
Related
I have a Result object that is tagged with "one" and "two". When I try to query for objects tagged "one" and "two", I get nothing back:
q = Result.objects.filter(Q(tags__name="one") & Q(tags__name="two"))
print len(q)
# prints zero, was expecting 1
Why does it not work with Q? How can I make it work?
The way django-taggit implements tagging is essentially through a ManytoMany relationship. In such cases there is a separate table in the database that holds these relations. It is usually called a "through" or intermediate model as it connects the two models. In the case of django-taggit this is called TaggedItem. So you have the Result model which is your model and you have two models Tag and TaggedItem provided by django-taggit.
When you make a query such as Result.objects.filter(Q(tags__name="one")) it translates to looking up rows in the Result table that have a corresponding row in the TaggedItem table that has a corresponding row in the Tag table that has the name="one".
Trying to match for two tag names would translate to looking up up rows in the Result table that have a corresponding row in the TaggedItem table that has a corresponding row in the Tag table that has both name="one" AND name="two". You obviously never have that as you only have one value in a row, it's either "one" or "two".
These details are hidden away from you in the django-taggit implementation, but this is what happens whenever you have a ManytoMany relationship between objects.
To resolve this you can:
Option 1
Query tag after tag evaluating the results each time, as it is suggested in the answers from others. This might be okay for two tags, but will not be good when you need to look for objects that have 10 tags set on them. Here would be one way to do this that would result in two queries and get you the result:
# get the IDs of the Result objects tagged with "one"
query_1 = Result.objects.filter(tags__name="one").values('id')
# use this in a second query to filter the ID and look for the second tag.
results = Result.objects.filter(pk__in=query_1, tags__name="two")
You could achieve this with a single query so you only have one trip from the app to the database, which would look like this:
# create django subquery - this is not evaluated, but used to construct the final query
subquery = Result.objects.filter(pk=OuterRef('pk'), tags__name="one").values('id')
# perform a combined query using a subquery against the database
results = Result.objects.filter(Exists(subquery), tags__name="two")
This would only make one trip to the database. (Note: filtering on sub-queries requires django 3.0).
But you are still limited to two tags. If you need to check for 10 tags or more, the above is not really workable...
Option 2
Query the relationship table instead directly and aggregate the results in a way that give you the object IDs.
# django-taggit uses Content Types so we need to pick up the content type from cache
result_content_type = ContentType.objects.get_for_model(Result)
tag_names = ["one", "two"]
tagged_results = (
TaggedItem.objects.filter(tag__name__in=tag_names, content_type=result_content_type)
.values('object_id')
.annotate(occurence=Count('object_id'))
.filter(occurence=len(tag_names))
.values_list('object_id', flat=True)
)
TaggedItem is the hidden table in the django-taggit implementation that contains the relationships. The above will query that table and aggregate all the rows that refer either to the "one" or "two" tags, group the results by the ID of the objects and then pick those where the object ID had the number of tags you are looking for.
This is a single query and at the end gets you the IDs of all the objects that have been tagged with both tags. It is also the exact same query regardless if you need 2 tags or 200.
Please review this and let me know if anything needs clarification.
first of all, this three are same:
Result.objects.filter(tags__name="one", tags__name="two")
Result.objects.filter(Q(tags__name="one") & Q(tags__name="two"))
Result.objects.filter(tags__name_in=["one"]).filter(tags__name_in=["two"])
i think the name field is CharField and no record could be equal to "one" and "two" at same time.
in python code the query looks like this(always false, and why you are geting no result):
from random import choice
name = choice(["abtin", "shino"])
if name == "abtin" and name == "shino":
we use Q object for implement OR or complex queries
Into the example that works you do an end on two python objects (query sets). That gets applied to any record not necessarily to the same record that has one AND two as tag.
ps: Why do you use the in filter ?
q = Result.objects.filter(tags_name_in=["one"]).filter(tags_name_in=["two"])
add .distinct() to remove duplicates if expecting more than one unique object
I have a Product entity and a Shop entity.
A shop can have 0 to n products and a product can be in only one shop.
The product entity table is thus refering to the Shop entity through a shop_id table field.
When querying for the products of a given shop using doctrine query builder, we can do this:
$products = $this->getDoctrine()->getRepository('MyBundle:Product')
->createQueryBuilder('p')
->where('p.shop = :shop')
->setParameter('shop', $shop) // here we pass a shop object
->getQuery()->getResult();
or this:
$products = $this->getDoctrine()->getRepository('MyBundle:Product')
->createQueryBuilder('p')
->where('p.shop = :shopId')
->setParameter('shopId', $shopId) // we pass directly the shop id
->getQuery()->getResult();
And the both seem to work... I'm thus wondering: can we always pass directly entity ids instead of entity instances in such cases (ie: on a doctrine entity field that refers to another entity)?
I initially thought that only the first example would work...
According to the Doctrine documentation, you can pass the object to setParameter() if the object is managed.
extract from the documentation:
Calling setParameter() automatically infers which type you are setting as value. This works for integers, arrays of strings/integers, DateTime instances and for managed entities.
for more information, please see:
http://docs.doctrine-project.org/projects/doctrine-orm/en/latest/reference/query-builder.html#binding-parameters-to-your-query
I might be wrong, but I think that if you pass object instance instead of id number, Doctrine will automatically call $instance->getId() making your two queries the same when translated into SQL (even DQL).
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.
I wanted to know is there anything equivalent to:
select columnname from tablename
Like Django tutorial says:
Entry.objects.filter(condition)
fetches all the objects with the given condition. It is like:
select * from Entry where condition
But I want to make a list of only one column [which in my case is a foreign key]. Found that:
Entry.objects.values_list('column_name', flat=True).filter(condition)
does the same. But in my case the column is a foreign key, and this query loses the property of a foreign key. It's just storing the values. I am not able to make the look-up calls.
Of course, values and values_list will retrieve the raw values from the database. Django can't work its "magic" on a model which means you don't get to traverse relationships because you're stuck with the id the foreign key is pointing towards, rather than the ForeignKey field.
If you need to filters those values, you could do the following (assuming column_name is a ForeignKey pointing to MyModel):
ids = Entry.objects.values_list('column_name', flat=True).filter(...)
my_models = MyModel.objects.filter(pk__in=set(ids))
Here's a documentation for values_list()
To restrict a query set to a specific column(s) you use .values(columname)
You should also probably add distinct to the end, so your query will end being:
Entry.objects.filter(myfilter).values(columname).distinct()
See: https://docs.djangoproject.com/en/dev/ref/models/querysets/#django.db.models.query.QuerySet.values
for more information
Depending on your answer in the comment, I'll come back and edit.
Edit:
I'm not certain if the approach is right one though. You can get all of your objects in a python list by getting a normal queryset via filter and then doing:
myobjectlist = map(lambda x: x.mycolumnname, myqueryset)
The only problem with that approach is if your queryset is large your memory use is going to be equally large.
Anyway, I'm still not certain on some of the specifics of the problem.
You have a model A with a foreign key to another model B, and you want to select the Bs which are referred to by some A. Is that right? If so, the query you want is just:
B.objects.filter(a__isnull = False)
If you have conditions on the corresponding A, then the query can be:
B.objects.filter(a__field1 = value1, a__field2 = value2, ...)
See Django's backwards relation documentation for an explanation of why this works, and the ForeignKey.related_name option if you want to change the name of the backwards relation.
Is it possible to map to methods instead of properties?
I have a Customer class with a method "GetOrders()" typeof "ReadOnlyCollection" with backing field "_orders" typeof "IList".
I tried in CustomerMap:
HasMany<Order>(Reveal.Member<Customer>("_orders"))
.KeyColumn("CustomerId").Cascade.All().Inverse().Not.LazyLoad();
But I get an exception when running the insert for a customer containing 1 order.
Cannot insert the value NULL into column 'CustomerId', table 'Order';
column does not allow nulls. INSERT fails.
Does mapping methods (or at least their backing fields) not work?
Or am I doing something else wrong?
The problem was that I did not map the Customer to the Orders also.