I want to execute the following subquery in flask-SQLAlchemy but don't know how:
SELECT *
FROM (
SELECT *
FROM `articles`
WHERE publisher_id = "bild"
ORDER BY date_time DESC
LIMIT 10
) AS t
ORDER BY RAND( )
LIMIT 2
I know I can build the query as:
subq = Article.query.filter(Article.publisher_id =='bild').order_by(Article.date_time.desc()).limit(10).subquery()
qry = subq.select().order_by(func.rand()).limit(2)
However I don't know how to execute it in the same fashion as I would execute e.g.
articles = Article.query.filter(Article.publisher_id =='bild').all()
i.e. to get all the Article objects. What I can do is call
db.session.execute(qry).fetchall()
but this only gives me a list with actual row values instead of the objects on which I could for example call another function (like article.to_json()).
Any ideas? qry is a sqlalchemy.sql.selectable.Select object and db.session.execute(qry) a sqlalchemy.engine.result.ResultProxy while Article.query, on which I could call all(), is a flask_sqlalchemy.BaseQuery. Thanks!!
You can use select_entity_from
qry = db.session.query(Article).select_entity_from(subq).order_by(func.rand()).limit(2)
or from_self
Article.query.filter(Article.publisher_id =='bild')\
.order_by(Article.date_time.desc())\
.limit(10)\
.from_self()\
.order_by(func.rand())\
.limit(2)
Related
I have 2 tables (say Student and College) and a third table (StudentCollege), which has student_id and college_id foreign keys.
I want to give output from below query:
list = (
db.session.query(StudentCollegeModel.College_id)
.filter(StudentCollegeModel.student_id== student_id)
.all()
)
to below query:
(
db.session.query(CollegeModel)
.filter(CollegeModel.College_id.in_(list))
.all()
)
But its giving programming error.
You don't need to execute the first query to use it as a subquery in the second. This saves you from having to construct an in memory list of all of the College_id's before making the in_() query and means that you only make one round trip to the database.
subquery = (
db.session.query(StudentCollegeModel.College_id)
.filter(StudentCollegeModel.student_id== student_id)
)
result = (
db.session.query(CollegeModel)
.filter(CollegeModel.College_id.in_(subquery))
.all()
)
Your first query returns a list of named tuples. You need to convert it to a list. For example:
college_tuples = db.session.query(StudentCollegeModel.College_id).filter(StudentCollegeModel.student_id == student_id).all()
college_id_list = [r.College_id for r in college_tuples]
db.session.query(CollegeModel.filter(CollegeModel.College_id.in_(college_id_list)).all()
Here we fetch your college id tuples, flatten it and used the flattened version to preform your second query.
Similar as follows:
cs = StudentCollegeModel.query.filter_by(student_id=student_id).distinct('college_id').entities_with('college_id', flat=True).all()
CollegeModel.query.filter(CollegeModel.College_id.in_(cs)).all()
I needed to achieve something like this in Django ORM :
(SELECT * FROM `stats` WHERE MODE = 1 ORDER BY DATE DESC LIMIT 2)
UNION
(SELECT * FROM `stats` WHERE MODE = 2 ORDER BY DATE DESC LIMIT 2)
UNION
(SELECT * FROM `stats` WHERE MODE = 3 ORDER BY DATE DESC LIMIT 2)
UNION
(SELECT * FROM `stats` WHERE MODE = 6 ORDER BY DATE DESC LIMIT 2)
UNION
(SELECT * FROM `stats` WHERE MODE = 5 AND is_completed != 3 ORDER BY DATE DESC)
# mode 5 can return more than 100 records so NO LIMIT here
for which i wrote this :
query_run_now_job_ids = Stats.objects.filter(mode=5).exclude(is_completed=3).order_by('-date')
list_of_active_job_ids = Stats.objects.filter(mode=1).order_by('-date')[:2].union(
Stats.objects.filter(mode=2).order_by('-date')[:2],
Stats.objects.filter(mode=3).order_by('-date')[:2],
Stats.objects.filter(mode=6).order_by('-date')[:2],
query_run_now_job_ids)
but somehow list_of_active_job_ids returned is unordered i.e list_of_active_job_ids.ordered returns False due to which when this query is passed to Paginator class it gives :
UnorderedObjectListWarning:
Pagination may yield inconsistent results with an unordered object_list
I have already set ordering in class Meta in models.py
class Meta:
ordering = ['-date']
Without paginator query works fine and page loads but using paginator , view never loads it keeps on loading .
Is there any better alternate for achieving this without using chain of union .
So I tried another alternate for above mysql query but i'm stuck in another problem to write up condition for mode = 5 in this query :
SELECT
MODE ,
SUBSTRING_INDEX(GROUP_CONCAT( `job_id` SEPARATOR ',' ),',',2) AS job_id_list,
SUBSTRING_INDEX(GROUP_CONCAT( `total_calculations` SEPARATOR ',' ),',',2) AS total_calculations
FROM `stats`
ORDER BY DATE DESC
Even if I was able to write this Query it would lead me to another challenging situation i.e to convert this query for Django ORM .
So why My Query is not ordered even when i have set it in Class Meta .
Also if not this query , Is there any better alternate for achieving this ?
Help would be appreciated ! .
I'm using Python 2.7 and Django 1.11 .
While subqueries may be ordered, the resulting union data is not. You need to explicitly define the ordering.
from django.db import models
def make_query(mode, index):
return (
Stats.objects.filter(mode=mode).
annotate(_sort=models.Value(index, models.IntegerField())).
order_by('-date')
)
list_of_active_job_ids = make_query(1, 1)[:2].union(
make_query(2, 2)[:2],
make_query(3, 3)[:2],
make_query(6, 4)[:2],
make_query(5, 5).exclude(is_completed=3)
).order_by('_sort', '-date')
All I did was add a new, literal value field _sort that has a different value for each subquery and then ordered by it in the final query.The rest of the code is just to reduce duplication. It would have been even cleaner if it wasn't for that mode=6 subquery.
I'm trying to translate this SQL query into a Flask-SQLAlchemy call:
SELECT *
FROM "ENVOI"
WHERE "ID_ENVOI" IN (SELECT d."ID_ENVOI"
FROM "DECLANCHEMENT" d
WHERE d."STATUS" = 0
AND d."DATE" = (SELECT max("DECLANCHEMENT"."DATE")
FROM "DECLANCHEMENT"
WHERE "DECLANCHEMENT"."ID_ENVOI" = d."ID_ENVOI"))
As you can see, it uses subqueries and, most important part, one of the subqueries is a correlated query (it use d table defined in an outer query).
I know how to use subqueries with subquery() function, but I can't find documentation about correlated queries with SQLAlchemy. Do you know a way to do it ?
Yes, we can.
Have a look at the following example (especially the correlate method call):
from sqlalchemy import select, func, table, Column, Integer
table1 = table('table1', Column('col', Integer))
table2 = table('table2', Column('col', Integer))
subquery = select(
[func.if_(table1.c.col == 1, table2.c.col, None)]
).correlate(table1)
query = (
select([table1.c.col,
subquery.label('subquery')])
.select_from(table1)
)
if __name__ == '__main__':
print(query)
will result in the following query
SELECT table1.col, (SELECT if(table1.col = :col_1, table2.col, NULL) AS if_1
FROM table2) AS subquery
FROM table1
As you can see, if you call correlate on a select, the given Table will not be added to it's FROM-clause.
You have to do this even when you specify select_from directly, as SQLAlchemy will happily add any table it finds in the columns.
Based on the link from univerio's comment, I've done this code for my request:
Declch = db.aliased(Declanchement)
maxdate_sub = db.select([db.func.max(Declanchement.date)])\
.where(Declanchement.id_envoi == Declch.id_envoi)
decs_sub = db.session.query(Declch.id_envoi)\
.filter(Declch.status == SMS_EN_ATTENTE)\
.filter(Declch.date < since)\
.filter(Declch.date == maxdate_sub).subquery()
envs = Envoi.query.filter(Envoi.id_envoi.in_(decs_sub)).all()
I need a litle help with expressing in SQLAlchemy language my code like this:
SELECT
s.agent_id,
s.property_id,
p.address_zip,
(
SELECT v.valuation
FROM property_valuations v WHERE v.zip_code = p.address_zip
ORDER BY ABS(DATEDIFF(v.as_of, s.date_sold))
LIMIT 1
) AS back_valuation,
FROM sales s
JOIN properties p ON s.property_id = p.id
Inner subquery aimed to get property value from table propert_valuations with columns (zip_code INT, valuation DECIMAL, as_if DATE) closest to the date of sale from table sales. I know how to rewrite it but I completely stuck on order_by expression - I cannot prepare subquery to pass ordering member later.
Currently I have following queries:
subquery = (
session.query(PropertyValuation)
.filter(PropertyValuation.zip_code == Property.address_zip)
.order_by(func.abs(func.datediff(PropertyValuation.as_of, Sale.date_sold)))
.limit(1)
)
query = session.query(Sale).join(Sale.property_)
How to combine these queries together?
How to combine these queries together?
Use as_scalar(), or label():
subquery = (
session.query(PropertyValuation.valuation)
.filter(PropertyValuation.zip_code == Property.address_zip)
.order_by(func.abs(func.datediff(PropertyValuation.as_of, Sale.date_sold)))
.limit(1)
)
query = session.query(Sale.agent_id,
Sale.property_id,
Property.address_zip,
# `subquery.as_scalar()` or
subquery.label('back_valuation'))\
.join(Property)
Using as_scalar() limits returned columns and rows to 1, so you cannot get the whole model object using it (as query(PropertyValuation) is a select of all the attributes of PropertyValuation), but getting just the valuation attribute works.
but I completely stuck on order_by expression - I cannot prepare subquery to pass ordering member later.
There's no need to pass it later. Your current way of declaring the subquery is fine as it is, since SQLAlchemy can automatically correlate FROM objects to those of an enclosing query. I tried creating models that somewhat represent what you have, and here's how the query above works out (with added line-breaks and indentation for readability):
In [10]: print(query)
SELECT sale.agent_id AS sale_agent_id,
sale.property_id AS sale_property_id,
property.address_zip AS property_address_zip,
(SELECT property_valuations.valuation
FROM property_valuations
WHERE property_valuations.zip_code = property.address_zip
ORDER BY abs(datediff(property_valuations.as_of, sale.date_sold))
LIMIT ? OFFSET ?) AS back_valuation
FROM sale
JOIN property ON property.id = sale.property_id
I'd like to know if it's possible to generate a SELECT COUNT(*) FROM TABLE statement in SQLAlchemy without explicitly asking for it with execute().
If I use:
session.query(table).count()
then it generates something like:
SELECT count(*) AS count_1 FROM
(SELECT table.col1 as col1, table.col2 as col2, ... from table)
which is significantly slower in MySQL with InnoDB. I am looking for a solution that doesn't require the table to have a known primary key, as suggested in Get the number of rows in table using SQLAlchemy.
Query for just a single known column:
session.query(MyTable.col1).count()
I managed to render the following SELECT with SQLAlchemy on both layers.
SELECT count(*) AS count_1
FROM "table"
Usage from the SQL Expression layer
from sqlalchemy import select, func, Integer, Table, Column, MetaData
metadata = MetaData()
table = Table("table", metadata,
Column('primary_key', Integer),
Column('other_column', Integer) # just to illustrate
)
print select([func.count()]).select_from(table)
Usage from the ORM layer
You just subclass Query (you have probably anyway) and provide a specialized count() method, like this one.
from sqlalchemy.sql.expression import func
class BaseQuery(Query):
def count_star(self):
count_query = (self.statement.with_only_columns([func.count()])
.order_by(None))
return self.session.execute(count_query).scalar()
Please note that order_by(None) resets the ordering of the query, which is irrelevant to the counting.
Using this method you can have a count(*) on any ORM Query, that will honor all the filter andjoin conditions already specified.
I needed to do a count of a very complex query with many joins. I was using the joins as filters, so I only wanted to know the count of the actual objects. count() was insufficient, but I found the answer in the docs here:
http://docs.sqlalchemy.org/en/latest/orm/tutorial.html
The code would look something like this (to count user objects):
from sqlalchemy import func
session.query(func.count(User.id)).scalar()
Addition to the Usage from the ORM layer in the accepted answer: count(*) can be done for ORM using the query.with_entities(func.count()), like this:
session.query(MyModel).with_entities(func.count()).scalar()
It can also be used in more complex cases, when we have joins and filters - the important thing here is to place with_entities after joins, otherwise SQLAlchemy could raise the Don't know how to join error.
For example:
we have User model (id, name) and Song model (id, title, genre)
we have user-song data - the UserSong model (user_id, song_id, is_liked) where user_id + song_id is a primary key)
We want to get a number of user's liked rock songs:
SELECT count(*)
FROM user_song
JOIN song ON user_song.song_id = song.id
WHERE user_song.user_id = %(user_id)
AND user_song.is_liked IS 1
AND song.genre = 'rock'
This query can be generated in a following way:
user_id = 1
query = session.query(UserSong)
query = query.join(Song, Song.id == UserSong.song_id)
query = query.filter(
and_(
UserSong.user_id == user_id,
UserSong.is_liked.is_(True),
Song.genre == 'rock'
)
)
# Note: important to place `with_entities` after the join
query = query.with_entities(func.count())
liked_count = query.scalar()
Complete example is here.
If you are using the SQL Expression Style approach there is another way to construct the count statement if you already have your table object.
Preparations to get the table object. There are also different ways.
import sqlalchemy
database_engine = sqlalchemy.create_engine("connection string")
# Populate existing database via reflection into sqlalchemy objects
database_metadata = sqlalchemy.MetaData()
database_metadata.reflect(bind=database_engine)
table_object = database_metadata.tables.get("table_name") # This is just for illustration how to get the table_object
Issuing the count query on the table_object
query = table_object.count()
# This will produce something like, where id is a primary key column in "table_name" automatically selected by sqlalchemy
# 'SELECT count(table_name.id) AS tbl_row_count FROM table_name'
count_result = database_engine.scalar(query)
I'm not clear on what you mean by "without explicitly asking for it with execute()" So this might be exactly what you are not asking for.
OTOH, this might help others.
You can just run the textual SQL:
your_query="""
SELECT count(*) from table
"""
the_count = session.execute(text(your_query)).scalar()
def test_query(val: str):
query = f"select count(*) from table where col1='{val}'"
rtn = database_engine.query(query)
cnt = rtn.one().count
but you can find the way if you checked debug watch
query = session.query(table.column).filter().with_entities(func.count(table.column.distinct()))
count = query.scalar()
this worked for me.
Gives the query:
SELECT count(DISTINCT table.column) AS count_1
FROM table where ...
Below is the way to find the count of any query.
aliased_query = alias(query)
db.session.query(func.count('*')).select_from(aliased_query).scalar()
Here is the link to the reference document if you want to explore more options or read details.