Peter Vandenabeele wrote in post #1052874:
> A custom validation on the compound unique key would behave
> similar, always do a SELECT (and suffer from the race condition
> documented in
>
http://api.rubyonrails.org/classes/ActiveRecord/Validations/ClassMethods.html#method-i-validates_uniqueness_of
> ).
I was aware of the race condition in the original post, which is why I
thought this might be a 'clever' implementation. To merge what @Peter
said, I guess you can summarize it as:
If the likelihood of duplicate entries is low, let the db raise an error
-- this saves a SELECT call for non-duplicate entries:
def self.create_relation(src, dst)
create!(:src_id => src.id, :dst_id => dst.id)
rescue ActiveRecord::RecordNotUnique
where(:src_id => src.id, :dst_id => dst.id)
end
If the likelihood of duplicate entries is high, use first_or_create as a
first line of defense. This generates an extra SELECT, but (presumably)
that's cheaper than frequent calls to raise/rescue[*]. Regardless, keep
the rescue clause to avoid race conditions:
def self.create_relation(src, dst)
where(:src_id => src.id, :dst_id => dst.id).first_or_create!
rescue ActiveRecord::RecordNotUnique
where(:src_id => src.id, :dst_id => dst.id)
end
[*] This begs for a benchmarking test to compare the cost of a SELECT vs
raise/rescue. I'll put it on the list.
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