only issue a warning if column doesn't exist

app/models/concerns/sha_attribute.rb

@@ -22,7 +22,8 @@ module ShaAttribute
       column = columns.find { |c| c.name == name.to_s }
 
       unless column
-        raise ArgumentError.new("sha_attribute #{name.inspect} is invalid since the column doesn't exist")
+        warn "WARNING: sha_attribute #{name.inspect} is invalid since the column doesn't exist - you may need to run database migrations"
+        return
       end
 
       unless column.type == :binary

spec/models/concerns/sha_attribute_spec.rb

@@ -36,24 +36,26 @@ describe ShaAttribute do
       end
 
       context 'when the table does not exist' do
-        it 'allows the attribute to be added' do
+        it 'allows the attribute to be added and issues a warning' do
           allow(model).to receive(:table_exists?).and_return(false)
 
           expect(model).not_to receive(:columns)
           expect(model).to receive(:attribute)
+          expect(model).to receive(:warn)
 
           model.sha_attribute(:name)
         end
       end
 
       context 'when the column does not exist' do
-        it 'raises ArgumentError' do
+        it 'allows the attribute to be added and issues a warning' do
           allow(model).to receive(:table_exists?).and_return(true)
 
           expect(model).to receive(:columns)
-          expect(model).not_to receive(:attribute)
+          expect(model).to receive(:attribute)
+          expect(model).to receive(:warn)
 
-          expect { model.sha_attribute(:no_name) }.to raise_error(ArgumentError)
+          model.sha_attribute(:no_name)
         end
       end