With the following Django model:
class Item(models.Model):
name = CharField(max_len=256)
description = TextField()
I need to formulate a filter method that takes a list of n words (word_list) and returns the queryset of Items where each word in word_list can be found, either in the name or the description.
To do this with a single field is straightforward enough. Using the reduce technique described here (this could also be done with a for loop), this looks like:
q = reduce(operator.and_, (Q(description__contains=word) for word in word_list))
Item.objects.filter(q)
I want to do the same thing but take into account that each word can appear either in the name or the description. I basically want to query the concatenation of the two fields, for each word. Can this be done?
I have read that there is a concatenation operator in Postgresql, || but I am not sure if this can be utilized somehow in django to achieve this end.
As a last resort, I can create a third column that contains the combination of the two fields and maintain it via post_save signal handlers and/or save method overrides, but I'm wondering whether I can do this on the fly without maintaining this type of "search index" type of column.
The most straightforward way would be to use Q to do an OR:
lookups = [Q(name__contains=word) | Q(description__contains=word)
for word in words]
Item.objects.filter(*lookups) # the same as and'ing them together
I can't speak to the performance of this solution as compared to your other two options (raw SQL concatenation or denormalization), but it's definitely simpler.
Related
I have a queryset like this:
predicts = Prediction.objects.select_related('match').filter(match_id=pk)
I need to annotate this with a new field is_correct. I need to compare two string fields and the result should be annotated in this new field. the fields that I want to compare are:
predict from Prediction table
result from Match table (that has been joined through select_related)
I need to know what expression should I put inside my annotate function; below I have my current code which throughs a TypeError exception:
predicts = predicts.annotate(is_correct=(F('predict') == F('result')))
all help will be greatly appreciated.
UPDATE:
I found an alternative solution that does the job for me (filtering the Prediction based on Match result using filter and exclude), but I still like to know how to address this specific case where the new annotated field is the result of the comparison between two other fields of the queryset. For those who may need it, in Django 2.2 and later the Nullif database function does a comparison between two fields.
You can use the extra function, a hook for injecting specific clauses into the SQL.
First of all, we must know the names of the apps and the models, or the name of the tables in the database.
Assuming that in your case, the two tables are called "app_prediction" and "app_match".
The sentence would be as follows:
Prediction.objects.select_related('match').extra(
select={'is_correct': "app_prediction.predict = app_match.result"}
)
This will add a field called is_correct in your result,
in the database, the fields and tables must be called in the same way.
It would be best to see the models.
I'm implementing search functionality with an option of looking for a record by matching multiple tables and multiple fields in these tables.
Say I want to find a Customer by his/her first or last name, or by ID of placed Order which is stored in different model than Customer.
The easy scenario which I already implemented is that a user only types single word into search field, I then use Django Q to query Order model using direct field reference or related_query_name reference like:
result = Order.objects.filter(
Q(customer__first_name__icontains=user_input)
|Q(customer__last_name__icontains=user_input)
|Q(order_id__icontains=user_input)
).distinct()
Piece of a cake, no problems at all.
But what if user wants to narrow the search and types multiple words into search field.
Example: user has typed Bruce and got a whole lot of records back as a result of search.
Now he/she wants to be more specific and adds customer's last name to search.So the search becomes Bruce Wayne, after splitting this into separate parts I'm having Bruce and Wayne. Obviously I don't want to search Orders model because order_id is a single-word instance and it's sufficient to find customer at once so for this case I'm dropping it out of query at all.
Now I'm trying to match customer by both first AND last name, I also want to handle the scenario where the order of provided data is random, to properly handle Bruce Wayne and Wayne Bruce, meaning I still have customers full name but the position of first and last name aren't fixed.
And this is the question I'm looking answer for: how to build query that will search multiple fields of model not knowing which of search words belongs to which table.
I'm guessing the solution is trivial and there's for sure an elegant way to create such a dynamic query, but I can't think of a way how.
You can dynamically OR a variable number of Q objects together to achieve your desired search. The approach below makes it trivial to add or remove fields you want to include in the search.
from functools import reduce
from operator import or_
fields = (
'customer__first_name__icontains',
'customer__last_name__icontains',
'order_id__icontains'
)
parts = []
terms = ["Bruce", "Wayne"] # produce this from your search input field
for term in terms:
for field in fields:
parts.append(Q(**{field: term}))
query = reduce(or_, parts)
result = Order.objects.filter(query).distinct()
The use of reduce combines the Q objects by ORing them together. Credit to that part of the answer goes to this answer.
The solution I came up with is rather complex, but it works exactly the way I wanted to handle this problem:
search_keys = user_input.split()
if len(search_keys) > 1:
first_name_set = set()
last_name_set = set()
for key in search_keys:
first_name_set.add(Q(customer__first_name__icontains=key))
last_name_set.add(Q(customer__last_name__icontains=key))
query = reduce(and_, [reduce(or_, first_name_set), reduce(or_, last_name_set)])
else:
search_fields = [
Q(customer__first_name__icontains=user_input),
Q(customer__last_name__icontains=user_input),
Q(order_id__icontains=user_input),
]
query = reduce(or_, search_fields)
result = Order.objects.filter(query).distinct()
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
Given a model with both Boolean and TextField fields, I want to do a query that finds records that match some criteria AND have more than "n" words in the TextField. Is this possible? e..g.:
class Item(models.Model):
...
notes = models.TextField(blank=True,)
has_media = models.BooleanField(default=False)
completed = models.BooleanField(default=False)
...
This is easy:
items = Item.objects.filter(completed=True,has_media=True)
but how can I filter for a subset of those records where the "notes" field has more than, say, 25 words?
Try this:
Item.objects.extra(where=["LENGTH(notes) - LENGTH(REPLACE(notes, ' ', ''))+1 > %s"], params=[25])
This code uses Django's extra queryset method to add a custom WHERE clause. The calculation in the WHERE clause basically counts the occurances of the "space" character, assuming that all words are prefixed by exactly one space character. Adding one to the result accounts for the first word.
Of course, this calculation is only an approximation to the real word count, so if it has to be precise, I'd do the word count in Python.
I dont know what SQL need to be run in order for the DB to do the work, which is really what we want, but you can monkey-patch it.
Make an extra fields named wordcount or something, then extend the save method and make it count all the words in notes before saving the model.
The it is trivial to loop over and there is still no chance that this denormalization of data will break since the save method is always run on save.
But there might be a better way, but if all else fails, this is what I would do.
I have a two models:
class Category(models.Model):
pass
class Item(models.Model):
cat = models.ForeignKey(Category)
I am trying to return all Categories for which all of that category's items belong to a given subset of item ids (fixed thanks). For example, all categories for which all of the items associated with that category have ids in the set [1,3,5].
How could this be done using Django's query syntax (as of 1.1 beta)? Ideally, all the work should be done in the database.
Category.objects.filter(item__id__in=[1, 3, 5])
Django creates the reverse relation ship on the model without the foreign key. You can filter on it by using its related name (usually just the model name lowercase but it can be manually overwritten), two underscores, and the field name you want to query on.
lets say you require all items to be in the following set:
allowable_items = set([1,3,4])
one bruteforce solution would be to check the item_set for every category as so:
categories_with_allowable_items = [
category for category in
Category.objects.all() if
set([item.id for item in category.item_set.all()]) <= allowable_items
]
but we don't really have to check all categories, as categories_with_allowable_items is always going to be a subset of the categories related to all items with ids in allowable_items... so that's all we have to check (and this should be faster):
categories_with_allowable_items = set([
item.category for item in
Item.objects.select_related('category').filter(pk__in=allowable_items) if
set([siblingitem.id for siblingitem in item.category.item_set.all()]) <= allowable_items
])
if performance isn't really an issue, then the latter of these two (if not the former) should be fine. if these are very large tables, you might have to come up with a more sophisticated solution. also if you're using a particularly old version of python remember that you'll have to import the sets module
I've played around with this a bit. If QuerySet.extra() accepted a "having" parameter I think it would be possible to do it in the ORM with a bit of raw SQL in the HAVING clause. But it doesn't, so I think you'd have to write the whole query in raw SQL if you want the database doing the work.
EDIT:
This is the query that gets you part way there:
from django.db.models import Count
Category.objects.annotate(num_items=Count('item')).filter(num_items=...)
The problem is that for the query to work, "..." needs to be a correlated subquery that looks up, for each category, the number of its items in allowed_items. If .extra had a "having" argument, you'd do it like this:
Category.objects.annotate(num_items=Count('item')).extra(having="num_items=(SELECT COUNT(*) FROM app_item WHERE app_item.id in % AND app_item.cat_id = app_category.id)", having_params=[allowed_item_ids])