It's not possible to set ES replicas to 0

For a small cluster / development environment one might want to set "number_of_replicas": 0 (effectively disabling replication) for an ES index, but it's not possible because of the current validation

validates :elasticsearch_replicas,
          presence: true,
          numericality: { only_integer: true, greater_than: 0 }

Also, there is a problem with unassigned shards if we use replica on a single-node cluster