This ticket is for changing to use primary read preference for validates_uniqueness_off.

Note that per the comments in https://github.com/mongodb/mongoid/pull/5451, the only validator that Mongoid implements that queries the database is the uniquess one. Applications with custom validatiors should adjust read preference as necessary for their validators.

Original ticket
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The validates uniqueness check can yield a false positive if reading from secondary and there is replication lag (i.e. the secondary does not yet have the conflicting data.) It would be useful to have a global flag to force validates_uniqueness to use read mode: primary. In my opinion this is fine as a global, I don't think we need an option on the validation itself.