Add value to params before save

I'm not entirely clear what you are trying to achieve but why not set
the attribute directly then save? You don't need to add it to the
params hash, but just to the object itself (if I understand your
question correctly).

@my_object.attribute = value
if @my_object.save
# do something here
else
# do something else
end

You might want to also check out the 'update' and 'update_attributes'
methods as well.

...and the 'update_attribute' method as well.

update_attribute and update_attributes allow you to set the
attribute(s) and save the record in one step. So you might be using
the params hash to populate the bulk of the object's attributes and
then adding in extra attribute(s) with these methods.