How to add a WHERE clause in UPDATE query in Datamapper

Question

I have a Datamapper model, say XYZ. Now, There are multiple threads which, at times, read a row using this model and attempt to update the same row - only one should should succeed, depending on a property of XYZ, say abc

class XYZ
include DataMapper::Resource
property :id # primary key
property :abc
end

Now:

obj = XYZ.get(some_id)
obj.update(abc: 10) # Assume abc column value was 5 earlier

This may happen in several threads, and may happen simultaneously. Also, the new value for column abc is different in each thread. Once, when a thread updates abc, others should not. Essentially what I am trying to do is to run this query via datamapper:

UPDATE `xyz` SET `abc` = 20 WHERE `id` = <some id> AND `abc` = 5

the model.update function does not allows to update an attribute and put a condition of that attribute simultaneously. I know I can run an SQL query directly; but is there any other way?

Answer 1

I would do something like this:

XYZ.all( :id => some_id, :abc => 5 ).update( :abc => 20 )