Find enterprise relationships involving any of a number of enterprises

app/models/enterprise_relationship.rb

@@ -9,4 +9,8 @@ class EnterpriseRelationship < ActiveRecord::Base
     joins('LEFT JOIN enterprises AS parent_enterprises ON parent_enterprises.id = enterprise_relationships.parent_id').
     joins('LEFT JOIN enterprises AS child_enterprises ON child_enterprises.id = enterprise_relationships.child_id')
   scope :by_name, with_enterprises.order('parent_enterprises.name, child_enterprises.name')
+
+  scope :involving_enterprises, ->(enterprises) {
+    where('parent_id IN (?) OR child_id IN (?)', enterprises, enterprises)
+  }
 end

spec/models/enterprise_relationship_spec.rb

@@ -2,15 +2,32 @@ require 'spec_helper'
 
 describe EnterpriseRelationship do
   describe "scopes" do
+    let(:e1)  { create(:enterprise, name: 'A') }
+    let(:e2)  { create(:enterprise, name: 'B') }
+    let(:e3)  { create(:enterprise, name: 'C') }
+
     it "sorts by parent, child enterprise name" do
-      e1 = create(:enterprise, name: 'A')
-      e2 = create(:enterprise, name: 'B')
-      e3 = create(:enterprise, name: 'C')
       er1 = create(:enterprise_relationship, parent: e1, child: e3)
       er2 = create(:enterprise_relationship, parent: e2, child: e1)
       er3 = create(:enterprise_relationship, parent: e1, child: e2)
 
       EnterpriseRelationship.by_name.should == [er3, er1, er2]
     end
+
+    describe "finding relationships involving some enterprises" do
+      let!(:er) { create(:enterprise_relationship, parent: e1, child: e2) }
+
+      it "returns relationships where an enterprise is the parent" do
+        EnterpriseRelationship.involving_enterprises([e1]).should == [er]
+      end
+
+      it "returns relationships where an enterprise is the child" do
+        EnterpriseRelationship.involving_enterprises([e2]).should == [er]
+      end
+
+      it "does not return other relationships" do
+        EnterpriseRelationship.involving_enterprises([e3]).should == []
+      end
+    end
   end
 end