rewrite for clarity and AR 5.1 compat

In AR 5.1 find_or_create_by and new_record_before_save? interact differently, to break the original code. (I think maybe it was relying on a bug in new_record_before_save?, I don't really understand what new_record_before_save? is).

I believe this is the same logic, rewritten more clearly and in a way that will work in Rails 5.1.

Note that find_or_create_by is NOT atomic, in any Rails version. So we should not have introduced any race condition that wasn't there already, we're still creating by soon after find if not present, just a slight intervening check for token.last == 0, shoudn't be much racier than before.

Author: jrochkind

lib/oai/provider/model/activerecord_caching_wrapper.rb

@@ -90,17 +90,16 @@ def next_set(token_string)
     # select a subset of the result set, and return it with a
     # resumption token to get the next subset
     def select_partial(token)
-      if 0 == token.last
-        oaitoken = OaiToken.find_or_create_by(token: token.to_s)
-        if oaitoken.new_record_before_save?
-          OaiToken.connection.execute("insert into " +
-            "#{OaiEntry.table_name} (oai_token_id, record_id) " +
-            "select #{oaitoken.id}, id from #{model.table_name} where " +
-            "#{OaiToken.sanitize_sql(token_conditions(token))}")
-        end
+      oaitoken = OaiToken.find_by(token: token.to_s)
+
+      if 0 == token.last && oaitoken.nil?
+        oaitoken = OaiToken.create!(token: token.to_s)
+        OaiToken.connection.execute("insert into " +
+          "#{OaiEntry.table_name} (oai_token_id, record_id) " +
+          "select #{oaitoken.id}, id from #{model.table_name} where " +
+          "#{OaiToken.sanitize_sql(token_conditions(token))}")
       end
 
-      oaitoken = OaiToken.find_by_token(token.to_s)
       raise ResumptionTokenException.new unless oaitoken
 
       PartialResult.new(