i want to construct a a pojo with some records from 2 tables using Hibernate criteria api. I'm constructing a ProjectionList with needed records from the 1st table, because i don't have a bidirectional relationship between the 2 tabls , have only from 2nd to first i've made a DetachedCriteria(subcriteria) and made a projection for the needed record.
i don't know how to add the subcriteria to the ProjectionList for having an array of the needed records
Criteria criteria = currentSession.createCriteria(getClazz());
projectionList.add(Projections.property("name"), "name");
projectionList.add(Projections.property("str"), "street");
projectionList.add(Projections.property("nr"), "nr");
DetachedCriteria subcriteria=DetachedCriteria.forClass(B.class,"b");
subcriteria.createAlias("b.adress", "adr",CriteriaSpecification.LEFT_JOIN);
subcriteria.setProjection(Projections.property("adr.id"));
Thanks!!!
If the first class has for example addressId your query can be something like that:
crteria.add(Subqueries.propertyIn("addressId", subcriteria));
Related
I would like to order the returned outgoing nodes in a specific way, based on a relationship property. Can this customized at all? I can't even find anything in the docs about what the default ordering is.
#Relationship("CREATED")
private List<Node> nodes;
There is no default sorting in Spring Data Neo4j.
Also there is no default sorting in the database.
You should not make any assumptions if there is no explicit ordering in the query.
If you want to go the path in Cypher you have to define a custom query.
For this, I am referring to the movie graph (:play movies) to populate the database and have some example data.
If you would call:
MATCH (m:Movie{title:'The Matrix'}) OPTIONAL MATCH (m)<-[:ACTED_IN]-(p) return m, collect(p)
The actors will have a random order.
But if you add ordering by e.g. the actor's name to this, you get the desired result.
MATCH (m:Movie{title:'The Matrix'}) OPTIONAL MATCH (m)<-[:ACTED_IN]-(p) WITH m, p ORDER BY p.name return m, collect(p)
class A(models.Model)
results = models.TextField()
class B(models.Model)
name = models.CharField(max_length=20)
res = models.ManyToManyField(A)
Let's suppose we have above 2 models. A model has millions of objects.
I would like to know what would be the best efficient/fastest way to get all the results objects of a particular B object.
Let's suppose we have to retrieve all results for object number 5 of B
Option 1 : A.objects.filter(b__id=5)
(OR)
Option 2 : B.objects.get(id=5).res.all()
Option 1: My Question is filtering by id on A model objects would take lot of time? since there are millions of A model objects.
Option 2: Question: does res field on B model stores the id value of A model objects?
The reason why I'm assuming the option 2 would be a faster way since it stores the reference of A model objects & directly getting those object values first and making the second query to fetch the results. whereas in the first option filtering by id or any other field would take up a lot of time
The first expression will result in one database query. Indeed, it will query with:
SELECT a.*
FROM a
INNER JOIN a_b ON a_b.a_id = a.id
WHERE a_b.b_id = 5
The second expression will result in two queries. Indeed, first Django will query to fetch that specific B object with a query like:
SELECT b.*
FROM b
WHERE b.id = 5
then it will make exactly the same query to retrieve the related A objects.
But retrieving the A object is here not necessary (unless you of course need it somewhere else). You thus make a useless database query.
My Question is filtering by id on A model objects would take lot of time? since there are millions of A model objects.
A database normally stores an index on foreign key fields. This thus means that it will filter effectively. The total number of A objects is usually not (that) relevant (since it uses a datastructure to accelerate search like a B-tree [wiki]). The wiki page has a section named An index speeds the search that explains how this works.
Is it possible to prevent multiple querys when i use django ORM ? Example:
product = Product.objects.get(name="Banana")
for provider in product.providers.all():
print provider.name
This code will make 2 SQL querys:
1 - SELECT ••• FROM stock_product WHERE stock_product.name = 'Banana'
2 - SELECT stock_provider.id, stock_provider.name FROM stock_provider INNER JOIN stock_product_reference ON (stock_provider.id = stock_product_reference.provider_id) WHERE stock_product_reference.product_id = 1
I confess, i use Doctrine (PHP) for some projects. With doctrine it's possible to specify joins when retrieve the object (relations are populated in object, so no need to query database again for get attribute relation value).
Is it possible to do the same with Django's ORM ?
PS: I hop my question is comprehensive, english is not my primary language.
In Django 1.4 or later, you can use prefetch_related. It's like select_related but allows M2M relations and such.
product = Product.objects.prefetch_related('providers').get(name="Banana")
You still get two queries, though. From the docs:
prefetch_related, on the other hand, does a separate lookup for each relationship, and does the ‘joining’ in Python.
As for packing this down into a single query, Django won't do it like Doctrine because it doesn't do that much post-processing of the result set (Django would have to remove all the redundant column data, since you'll get a row per provider and each of these rows will have a copy of all of product's fields).
So if you want to pack this down to one query, you're going to have to turn it around and run the query on the Provider table (I'm guessing at your schema):
providers = Provider.objects.filter(product__name="Banana").select_related('product')
This should pack it down to one query, but you won't get a single product ORM object out of it, instead needing to get the product fields via providers[k].product.
You can use prefetch_related, sometimes in combination with select_related, to get all related objects in a single query: https://docs.djangoproject.com/en/1.5/ref/models/querysets/#prefetch-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).