[Rails] rake test:units error because not null constraint.

Hello.
I have this unit test:

test "User validations" do
user = User.new
assert user.invalid?
assert user.errors[:uid].any?
assert user.errors[:name].any?
assert user.errors[:role].any?
end

When I run it I have this error:

null value in column "uid" violates not-null constraint: INSERT INTO
"users" ("created_at", "updated_at", "id") VALUES ('2012-11-18
19:37:39', '2012-11-18 19:37:39', 298486374).
In the fixture user I've put:

one:
uid: "xxx.xxx"

Why the unit test got the error?
Thank you.

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