diff_mongoid_indexes.rb

defined_indexes = Model.index_specifications.map { |s| s.fields.map(&:to_s) };
existing_indexes = Model.collection.indexes.map { |i| i['key'].keys };

missing_indexes = defined_indexes - existing_indexes
# => []
extra_indexes = existing_indexes - defined_indexes - [['_id']]
# => []

#################################################################################

# if the arrays are correct (hopefully the indexes have {background: true} flag),
# then you're good to run

Model.create_indexes # or Model.remove_indexes