Raise an error if data could be lost from singularizing a list

When a property is set to `multiple: false` and we find more than one
statement for the predicate, then raise an
ActiveFedora::ConstraintError.

This should help prevent users from losing data that is not written using
ActiveFedora.

lib/active_fedora/attributes/property_builder.rb

@@ -27,7 +27,9 @@ def #{name}=(value)
     def self.define_singular_readers(mixin, name)
       mixin.class_eval <<-CODE, __FILE__, __LINE__ + 1
         def #{name}(*args)
-          get_values(:#{name}).first
+          vals = get_values(:#{name})
+          raise ActiveFedora::ConstraintError, "Expected \\"#{name}\\" to have 0-1 statements, but there are \#{vals.size}" if vals.size > 1
+          vals.first
         end
       CODE
     end

lib/active_fedora/errors.rb

@@ -54,6 +54,20 @@ class ReadOnlyRecord < ActiveFedoraError
   class IllegalOperation < ActiveFedoraError
   end
 
+  # Raised when the data has more than one statement for a predicate, but our constraints say it's singular
+  # This helps to prevent overwriting multiple values with a single value when round tripping:
+  #   class Book < ActiveFedora::Base
+  #     predicate :title, predicate: RDF::DC.title, multiple: false
+  #   end
+  #
+  #   b = Book.new
+  #   b.resource.title = ['foo', 'bar']
+  #   b.title # Raises ConstraintError
+  #   # which prevents us from doing:
+  #   b.title = b.title
+  class ConstraintError < ActiveFedoraError
+  end
+
   # Used to rollback a transaction in a deliberate way without raising an exception.
   # Transactions are currently incomplete
   class Rollback < ActiveFedoraError

spec/unit/attributes_spec.rb

@@ -418,6 +418,17 @@ class BarHistory4 < ActiveFedora::Base
             expect(subject[:abstract]).to be_nil
           end
 
+          context "when there are two assertions for the predicate" do
+            before do
+              subject.resource[:abstract] = ['foo', 'bar']
+            end
+            it "should raise an error if just returning the first value would cause data loss" do
+              expect { subject[:abstract] }.to raise_error ActiveFedora::ConstraintError, "Expected \"abstract\" to have 0-1 statements, but there are 2"
+            end
+          end
+        end
+
+        context "multiple values" do
           it "should return values" do
             expect(subject[:title]).to eq ['test1']
           end