CODEWITHSUNDEEP
pythonsqlalchemy
Liam M
Liam M

Reputation: 5432

How can I write an SQLAlchemy Query with a Join and an Aggregate?

I have a table that has 3 columns: type, content and time (an integer). For each 'type', I want to select the entry with the greatest (most recent) 'time' integer and the corresponding data. How can I do this using SQLAlchemy and Python? I could do this using SQL by performing:

select
  c.type,
  c.time,
  b.data
from
  parts as b

inner join

  (select
    a.type,
    max(a.time) as time
  from parts as a
  group by a.type) as c

on

b.type = c.type and
b.time = c.time

But how can I accomplish this in SQLAlchemy?

The table mapping:

class Structure(Base):
    __tablename__ = 'structure'
    id = Column(Integer, primary_key=True)
    type = Column(Text)
    content = Column(Text)
    time = Column(Integer)

    def __init__(self, type, content):
        self.type = type
        self.content = content
        self.time = time.time()

    def serialise(self):
        return {"type" : self.type,
            "content" : self.content};

The attempted query:

    max = func.max(Structure.time).alias("time")
    c = DBSession.query(max)\
        .add_columns(Structure.type, Structure.time)\
        .group_by(Structure.type)\
        .subquery()
    c.alias("c")

    b = DBSession.query(Structure.content)\
        .add_columns(c.c.type, c.c.time)\
        .join(c, Structure.type == c.c.type)

Gives me:

sqlalchemy.exc.OperationalError: (OperationalError) near "(": syntax error u'SELECT structure.content AS structure_content, anon_1.type AS anon_1_type, anon_1.t ime AS anon_1_time \nFROM structure JOIN (SELECT time.max_1 AS max_1, structure.type AS type, structure.time AS time \nFROM max(structure.time) AS time, structu re GROUP BY structure.type) AS anon_1 ON structure.type = anon_1.type' ()

I'm essentially stabbing in the dark, so any help would be appreciated.

Upvotes: 4

Views: 11248

Answers (1)

van
van

Reputation: 76972

Try the code below using sub-query:

subq = (session.query(
            Structure.type, 
            func.max(Structure.time).label("max_time")
        ).
        group_by(Structure.type)
        ).subquery()

qry = (session.query(Structure).
       join(subq, and_(Structure.type == subq.c.type, Structure.time == subq.c.max_time))
       )

print qry

producing SQL:

SELECT  structure.id AS structure_id, structure.type AS structure_type, structure.content AS structure_content, structure.time AS structure_time
FROM    structure 
JOIN    (SELECT structure.type AS type, max(structure.time) AS max_time
         FROM structure GROUP BY structure.type) AS anon_1 
    ON  structure.type = anon_1.type 
    AND structure.time = anon_1.max_time

Upvotes: 17

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