def nodes_from_evidence
      all_issues.
        includes(:evidence, evidence: :node).
        collect(&:evidence).
        # Each Issue can have 0, 1 or more Evidence
        map { |evidence_collection| evidence_collection.collect(&:node) }.
        flatten.
        uniq
    end
def nodes_from_evidence
      all_issues.
        includes(:evidence, evidence: :node).
        collect(&:evidence).
        # Each Issue can have 0, 1 or more Evidence
        map { |evidence_collection| evidence_collection.collect(&:node) }.
        flatten.
        uniq
    end
Private: this method returns a list of nodes associated with Evidence
 instances. When a node is affected by multiple issues, or multiple pieces
 of evidence, we just want a single reference to it.

 Returns and Array with a unique collection of Nodes.