Before_save and before_update not being called in tests

In one of the models I perform some operation. For simplicity let’s
assume
that I populate column ‘b’ with column ‘a’ * 2.

def before_update
self.b = self.a * 2
end

In my fixture I set values for a.

first:
id: 1
a: 10

Whe I run the test before_update is never invoked when rails populate
the
test table with the data from fixture. In development mode from the
browser
everything works fine.

Any idea why in test the berfore_* methods are not being invoked.

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