Minimum changes to avoid unnecessary DB lock on read

With accompanying tests to prove it.

lib/active_fedora/noid/minter/db.rb

@@ -6,14 +6,14 @@ module Noid
     module Minter
       class Db < Base
         def read
-          filtered_hash = instance.as_json.select { |key| %w(template counters seq rand namespace).include?(key) }
+          filtered_hash = instance.as_json.slice(*%w(template counters seq rand namespace))
           filtered_hash['counters'] = JSON.parse(filtered_hash['counters'], symbolize_names: true) if filtered_hash['counters']
           filtered_hash.symbolize_keys
         end
 
         def write!(minter)
           # namespace and template are the same, now update the other attributes
-          instance.update_attributes!(
+          instance(true).update_attributes!(
             seq: minter.seq,
             counters: JSON.generate(minter.counters),
             rand: Marshal.dump(minter.instance_variable_get(:@rand))
@@ -26,7 +26,7 @@ def write!(minter)
         # Should be fast enough to avoid terrible deadlock.
         # Must lock because of multi-connection context! (transaction is per connection -- not enough)
         # The DB table will only ever have at most one row per namespace.
-        # The 'default' namespace row is inserted by `rails generate active_fedora:noid:seed`.
+        # The 'default' namespace row is inserted by `rails generate active_fedora:noid:seed` or autofilled by instance below.
         # If you want another namespace, edit your config initialzer to something like:
         #     ActiveFedora::Noid.config.namespace = 'druid'
         #     ActiveFedora::Noid.config.template = '.reeedek'
@@ -43,10 +43,13 @@ def next_id
           id
         end
 
-        def instance
-          MinterState.lock.find_by!(
-            namespace: ActiveFedora::Noid.config.namespace,
-            template: ActiveFedora::Noid.config.template
+        # @param [Boolean] lock whether the returned object (row) should be locked for update or not
+        # @return [MinterState]
+        def instance(lock = false)
+          MinterState.find_by!(
+            { namespace: ActiveFedora::Noid.config.namespace,
+              template: ActiveFedora::Noid.config.template },
+            lock: lock
           )
         rescue ActiveRecord::RecordNotFound
           MinterState.seed!(

spec/unit/db_minter_spec.rb

@@ -43,6 +43,14 @@
     subject { db_minter.read }
 
     context 'when the database has been initialized' do
+      it 'does not lock the DB' do
+        expect(MinterState).not_to receive(:lock)
+        expect(MinterState).to receive(:find_by!).with(
+          anything,
+          hash_excluding(lock: true)
+        ).and_call_original
+        subject # have to call it to trigger above
+      end
       it 'has the expected namespace and template' do
         expect(subject).to include(namespace: ActiveFedora::Noid.config.namespace,
                                    template: ActiveFedora::Noid.config.template)