SQLAlchemy: How to group by two fields and filter by date

Question

So I have a table with a datestamp and two fields that I want to make sure that they are unique in the last month.

table.id
table.datestamp
table.field1
table.field2

There should be no duplicate record with the same field1 + 2 compound value in the last month.

The steps in my head are:

1. Group by the two fields
2. Look back over the last month's data to make sure this unique grouping doesn't occur.

I've got this far, but I don't think this works:

result = session.query(table).group_by(\
    table.field1,
    table.field2,
    func.month(table.timestamp))

But I'm unsure how to do this in sqlalchemy. Could someone advise me?

Thanks very much!

Solution

Following should point you in the right direction, also see inline comments:

qry = (session.query(
         table.c.field1,
         table.c.field2,    

        # #strftime* for year-month works on sqlite; 
            
        # @todo: find proper function for mysql (as in the question)
        # Also it is not clear if only MONTH part is enough, so that
        # May-2001 and May-2009 can be joined, or YEAR-MONTH must be used
        func.strftime('%Y-%m', table.c.datestamp),
        func.count(),
    )
    # optionally check only last 2 month data (could have partial months)
    .filter(table.c.datestamp < datetime.date.today() - datetime.timedelta(60))
    .group_by(
            table.c.field1,
            table.c.field2,
            func.strftime('%Y-%m', table.c.datestamp),
            )
    # comment this line out to see all the groups
    .having(func.count()>1)
  )

Comments

[0atman]

Thanks very much van, however your solution pokes holes in my sqlalchemy knowledge, what is the significance of the 'c' attribute of the table object?

[van]

if you have a table object, then c is a shortcut to columns. See SQL Expression Language Tutorial: sqlalchemy.org/docs/…

[0atman]

Don't worry, I should have just googled my question, as so often is the case!

[0atman]

However, I have realised that we're not using a native mysql timestamp, we are, in fact, using int(time.time()) to store the time, for compatibility with other systems. I'll modify my question to include this, perhaps I can add a check for only timestamps in the last 2,592,000 seconds? (30 days)

[0atman]

Van, I didn't notice you answered my question - I had the window still open and I should have F5'd before commenting. Thank you very much.

[Martin M.]

@van: I love you. It's so simple and clean I wouldn't even have thought of that...