feat: use a sequence for the version order on postgres

db/migrations/20201023_create_verification_number_sequence.rb

@@ -4,7 +4,7 @@
   up do
     if PactBroker::MigrationHelper.postgres?
       row = from(:verification_sequence_number).select(:value).limit(1).first
-      start = row ? row[:value] + 1 : 1
+      start = row ? row[:value] + 100 : 1
       run("CREATE SEQUENCE verification_number_sequence START WITH #{start}")
     end
   end

db/migrations/20201024_create_version_order_sequence.rb

@@ -0,0 +1,20 @@
+require_relative 'migration_helper'
+
+Sequel.migration do
+  up do
+    if PactBroker::MigrationHelper.postgres?
+      row = from(:version_sequence_number).select(:value).limit(1).first
+      start = row ? row[:value] + 100 : 1
+      run("CREATE SEQUENCE version_order_sequence START WITH #{start}")
+    end
+  end
+
+  down do
+    if PactBroker::MigrationHelper.postgres?
+      nextval = execute("SELECT nextval('version_order_sequence') as val") { |v| v.first["val"].to_i }
+      # Add a safety margin!
+      from(:version_sequence_number).update(value: nextval + 100)
+      run("DROP SEQUENCE version_order_sequence")
+    end
+  end
+end

lib/pact_broker/versions/sequence.rb

@@ -8,23 +8,27 @@ class Sequence < Sequel::Model(:version_sequence_number)
         # The easiest way to implement a cross database compatible sequence.
         # Sad, I know.
         def next_val
-          db.transaction do
-            for_update.first
-            select_all.update(value: Sequel[:value]+1)
-            row = first
-            if row
-              row.value
-            else
-              # The first row should have been created in the migration, so this code
-              # should only ever be executed in a test context, after we've truncated all the
-              # tables after a test.
-              # There would be a risk of a race condition creating two rows if this
-              # code executed in prod, as I don't think you can lock an empty table
-              # to prevent another record being inserted.
-              max_version_order = PactBroker::Domain::Version.max(:order)
-              value = max_version_order ? max_version_order + 100 : 1
-              insert(value: value)
-              value
+          if PactBroker::Repositories::Helpers.postgres?
+            db.execute("SELECT nextval('version_order_sequence') as val") { |v| v.first["val"].to_i }
+          else
+            db.transaction do
+              for_update.first
+              select_all.update(value: Sequel[:value]+1)
+              row = first
+              if row
+                row.value
+              else
+                # The first row should have been created in the migration, so this code
+                # should only ever be executed in a test context, after we've truncated all the
+                # tables after a test.
+                # There would be a risk of a race condition creating two rows if this
+                # code executed in prod, as I don't think you can lock an empty table
+                # to prevent another record being inserted.
+                max_version_order = PactBroker::Domain::Version.max(:order)
+                value = max_version_order ? max_version_order + 100 : 1
+                insert(value: value)
+                value
+              end
             end
           end
         end

tasks/database.rb

@@ -60,6 +60,7 @@ def drop_views
     def drop_sequences
       if psql?
         database.run('DROP SEQUENCE verification_number_sequence')
+        database.run('DROP SEQUENCE version_order_sequence')
       end
     end