Query that fetch messages for a specific user - sql

Question

I have a messages table like the following, how to fetch messages for a specific user, that is, fetch one most recent message with one user.For example, user1 has messages with user2 and user3,how to fetch one most recent message between user1 and user2, one most recent message between user1 and user3?
class Message(Base):
__tablename__ = 'messages'
id = Column(Integer(), primary_key=True)
sender_id = Column(Integer(), ForeignKey('users.id'))
body = Column(Text())
recipient_id = Column(Integer())
created_at = Column(DateTime())
updated_at = Column(DateTime())
The SQL I could figure out is following, but it is not enough.Could you help me? Thanks a lot,:).
session.query(Message).\
filter(or_(Message.user_id==user.id, Message.recipient_id==user.id))

Answer 1

Try
from sqlalchemy import desc
session.query(Message).\
filter(or_(Message.user_id==user.id, Message.recipient_id==user.id)).\
order_by(desc(Message.created_at)).\
first()

Answer 2

Code below should give you an example using created_at as the decision column for the latest. You can easily change it to be updated_at.
Note that for two users (A and B) it will return both latest messages: latest from A->B and latest B->A.
from datetime import date
from sqlalchemy import create_engine, Column, Text, Integer, DateTime, ForeignKey
from sqlalchemy.orm import sessionmaker, relationship
from sqlalchemy.sql import func, and_
from sqlalchemy.ext.declarative import declarative_base
engine = create_engine('sqlite:///:memory:', echo=True)
session = sessionmaker(bind=engine)()
Base = declarative_base(engine)
class User(Base):
__tablename__ = 'users'
id = Column(Integer(), primary_key=True)
name = Column(Text())
class Message(Base):
__tablename__ = 'messages'
id = Column(Integer(), primary_key=True)
sender_id = Column(Integer(), ForeignKey('users.id'))
body = Column(Text())
recipient_id = Column(Integer(), ForeignKey('users.id'))
created_at = Column(DateTime())
updated_at = Column(DateTime())
sender = relationship(User, backref="messages_sent", foreign_keys=sender_id)
recipient = relationship(User, backref="messages_recv", foreign_keys=recipient_id)
Base.metadata.create_all()
users = u1, u2, u3 = [
User(name='user1'),
User(name='user2'),
User(name='user3'),
]
session.add_all(users)
session.add_all([
Message(sender=u1, recipient=u2, body="bod12-a", created_at = date(2014, 1, 13)),
Message(sender=u1, recipient=u2, body="bod12-X", created_at = date(2014, 1, 3)),
Message(sender=u2, recipient=u3, body="bod23-X", created_at = date(2014, 1, 13)),
Message(sender=u2, recipient=u3, body="bod23-a", created_at = date(2013, 1, 13)),
])
session.flush()
subq = (session
.query(Message.sender_id, Message.recipient_id,
func.max(Message.created_at).label("max_created_at"))
.group_by(Message.sender_id, Message.recipient_id)
).subquery("subq")
q = (session
.query(Message)
.join(subq, and_(
Message.sender_id == subq.c.sender_id,
Message.recipient_id == subq.c.recipient_id,
Message.created_at == subq.c.max_created_at)
)
).all()
for x in q:
print(x.sender_id, x.recipient_id, x.body)