Backwards-compat for migration specs.

The specs are based on a schema version that doesn't know about
`internal_ids` table. However, the actual code being execute relies on
it.

app/models/internal_id.rb

@@ -17,6 +17,8 @@ class InternalId < ActiveRecord::Base
 
   validates :usage, presence: true
 
+  REQUIRED_SCHEMA_VERSION = 20180305095250
+
   # Increments #last_value and saves the record
   #
   # The operation locks the record and gathers a `ROW SHARE` lock (in PostgreSQL).
@@ -30,8 +32,22 @@ class InternalId < ActiveRecord::Base
 
   class << self
     def generate_next(subject, scope, usage, init)
+      # Shortcut if `internal_ids` table is not available (yet)
+      # This can be the case in other (unrelated) migration specs
+      return (init.call(subject) || 0) + 1 unless available?
+
       InternalIdGenerator.new(subject, scope, usage, init).generate
     end
+
+    def available?
+      @available_flag ||= ActiveRecord::Migrator.current_version >= REQUIRED_SCHEMA_VERSION # rubocop:disable Gitlab/PredicateMemoization
+    end
+
+    # Flushes cached information about schema
+    def reset_column_information
+      @available_flag = nil
+      super
+    end
   end
 
   class InternalIdGenerator
@@ -49,12 +65,12 @@ class InternalId < ActiveRecord::Base
     # subject: The instance we're generating an internal id for. Gets passed to init if called.
     # scope: Attributes that define the scope for id generation.
     # usage: Symbol to define the usage of the internal id, see InternalId.usages
-    # init: Block that gets called to initialize InternalId record if not yet present (optional)
+    # init: Block that gets called to initialize InternalId record if not present
     attr_reader :subject, :scope, :init, :scope_attrs, :usage
     def initialize(subject, scope, usage, init)
       @subject = subject
       @scope = scope
-      @init = init || ->(s) { 0 }
+      @init = init
       @usage = usage
 
       raise 'scope is not well-defined, need at least one column for scope (given: 0)' if scope.empty?

spec/models/internal_id_spec.rb

@@ -12,9 +12,9 @@ describe InternalId do
   end
 
   describe '.generate_next' do
-    context 'in the absence of a record' do
     subject { described_class.generate_next(issue, scope, usage, init) }
 
+    context 'in the absence of a record' do
       it 'creates a record if not yet present' do
         expect { subject }.to change { described_class.count }.from(0).to(1)
       end
@@ -47,6 +47,21 @@ describe InternalId do
       normalized = seq.map { |i| i - seq.min }
       expect(normalized).to eq((0..seq.size - 1).to_a)
     end
+
+    context 'with an insufficient schema version' do
+      before do
+        described_class.reset_column_information
+        expect(ActiveRecord::Migrator).to receive(:current_version).and_return(InternalId::REQUIRED_SCHEMA_VERSION - 1)
+      end
+
+      let(:init) { double('block') }
+
+      it 'calculates next internal ids on the fly' do
+        val = rand(1..100)
+        expect(init).to receive(:call).with(issue).and_return(val)
+        expect(subject).to eq(val + 1)
+      end
+    end
   end
 
   describe '#increment_and_save!' do