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Fix nessary indexes in code and recreate them:

rake db:mongoid:create_indexes RAILS_ENV=production

PS Before this I had to disable the errors when creating long indexes:

# mongo
MongoDB shell version:2.6.12
connecting to: test
> db.getSiblingDB('admin').runCommand({ setParameter:1, failIndexKeyTooLong:false})

Also may be needed reindex:

Also may be needed reindex:

db.getCollectionNames().forEach(function(collection){db[collection].reIndex()});