SQLObject: how to remove first N objects from the table?

Question

I have an application that keeps track on items and uses SQLObject python ORM. At some point of time app checks if table is not too big, and if it's more than 10000 items in table it removes first N items so it's 10000 items in the table. What is the elegant and effective way to write this down via SQLObject? All solutions i come after are slow and awkward :(.

Answer 1

Personally I have not used SQLOBJECT. So this is a simple delete statement based on the ordering records by inserted date, oldest N records will be removed and it's in MYSQL. Unable to set limit clause using a variable due to this bug, so I used the most direct method.

For this sampel I used the threshold as 6 as it's hard to have 10000...

• SQLFIDDLE

Sample table:

COL1    COL2
1       112
2       0
3       20
4       10
5       100
6       30
7       200
8       108

set @table_count:=0;
set @excess_count:=0;

select count(*) into @table_count
from tablex;

select (case when @table_count > 6
then (@table_count -6)
else 0 end) into @excess_count
;

select * 
from tablex
where col1 between 1 and @excess_count
order by col1 asc
;

Records selected:

COL1    COL2
1       112
2       0

Above query sorts records by col1 you may change it according to yours. So the delete query can be built upon the last select. So it's best to write a storedprocedure...as this is all one akward bunch of statements...

Answer 2

I know this thread is old, but I thought to post an entry in case someone comes across it via a Google search, just like I did.

I had a similar problem where I needed custom delete call. SQLObject gives you two options for creating a SQL DELETE with a custom where clause:

• Built it using the SQLBuilder classes (see http://sqlobject.org/SQLBuilder.html#delete)
• Call a SQL statement directly

I went with the second option as it was more flexible and streamlined:

dsqlMetricseries._connection.query("DELETE FROM dsql_metricseries WHERE metric_id = %s"%metric_id)

• dsqlMetricseries : SQLObject subclass, model class representing a DB table
• _connection : handle to SQLObject's internal DB connection
• query : SQLObject method for executing any SQL statement

Answer 3

As above, I realise this is an old post however other viewers may find this useful. I'm not sure how efficient this would be slicing the select (assuming record id is ascending as you add records).

myselect = tableobject.select(orderBy=tableobject.q.id)
if myselect.count() > 10000:
    tableobject.deleteMany(tableobject.q.id<myselect[10001].id)

Untested, but I routinely use deleteMany().