Delete ExplainerItem when ProjectMedia is destroyed. (#1995)

Fixes an issue reported by Sentry.

Fixes: CV2-5099.

app/models/concerns/project_media_associations.rb

@@ -21,7 +21,7 @@ module ProjectMediaAssociations
     has_one :claim_description, dependent: :destroy
     belongs_to :source, optional: true
     has_many :tipline_requests, as: :associated
-    has_many :explainer_items
+    has_many :explainer_items, dependent: :destroy
     has_many :explainers, through: :explainer_items
     has_annotations
   end

test/models/explainer_test.rb

@@ -110,4 +110,14 @@ def setup
     ex.description = 'Now this is the only paragraph'
     ex.save!
   end
+
+  test "should destroy explainer items when project media is destroyed" do
+    t = create_team
+    ex = create_explainer team: t
+    pm = create_project_media team: t
+    pm.explainers << ex
+    assert_difference 'ExplainerItem.count', -1 do
+      pm.destroy!
+    end
+  end
 end