Question

I recently wanted to write a simple migration script. I wrote:

@entries = Entries.all(:text => /test/)

@entries.each do |entry|
  entry.update(:text => entry.text.gsub!(/test/, "no-test"))
end

It didn’t save the records, even though the update statement returned true. What did I miss?

Answer 1

In the 1.x series of datamapper the dirty tracking is done via calling #== on the new and old attribute values to detect dirtyness. If an object is mutated inplace (for example with the String bang methods), the change cannot be detected as the “orignal” state gets mutated also.

Basically the following happens internally:

a = "foo"
b = a.gsub!("foo", "bar")
a == b     # => true both a and b refer to the same mutated object
a.equal?(b) # => true

In your example you assign the original mutated attribute back to the object, no identity change => no update detected.

In case you create a new object via String#gsub istead of mutating the original attribute value via String#gsub! you end up with a detectable change.

With assigning a new object with different value the following happens:

a = "foo"
b = a.gsub("foo", "bar")
a == b      # => false, loaded state does not equal resource state so change is detected
a.equal?(b) # => false

And for having all cases covered, assigning a new object with same value:

a = "foo"
b = "foo"
a == b      # => true, no dirtyness detected.
a.equal?(b) # => false

Hopefully this explains the semantic differences good enough to explain all similar cases.

BTW In datamapper 2.0 we have a differend mechanism that will also catch in place mutations. Disclaimer, I’m the author of this component called dm-session.