Add markdown cache columns to the database, but don't use them yet

This commit adds a number of _html columns and, with the exception of Note,
starts updating them whenever the content of their partner fields changes.

Note has a collision with the note_html attr_accessor; that will be fixed later

A background worker for clearing these cache columns is also introduced - use
`rake cache:clear` to set it off. You can clear the database or Redis caches
separately by running `rake cache:clear:db` or `rake cache:clear:redis`,
respectively.

1 files changed, 41 insertions, 0 deletions

diff --git a/config/initializers/ar5_batching.rb b/config/initializers/ar5_batching.rb
new file mode 100644
index 00000000000..35e8b3808e2
--- /dev/null
+++ b/config/initializers/ar5_batching.rb
@@ -0,0 +1,41 @@
+# Port ActiveRecord::Relation#in_batches from ActiveRecord 5.
+# https://github.com/rails/rails/blob/ac027338e4a165273607dccee49a3d38bc836794/activerecord/lib/active_record/relation/batches.rb#L184
+# TODO: this can be removed once we're using AR5.
+raise "Vendored ActiveRecord 5 code! Delete #{__FILE__}!" if ActiveRecord::VERSION::MAJOR >= 5
+
+module ActiveRecord
+  module Batches
+    # Differences from upstream: enumerator support was removed, and custom
+    # order/limit clauses are ignored without a warning.
+    def in_batches(of: 1000, start: nil, finish: nil, load: false)
+      raise "Must provide a block" unless block_given?
+
+      relation = self.reorder(batch_order).limit(of)
+      relation = relation.where(arel_table[primary_key].gteq(start)) if start
+      relation = relation.where(arel_table[primary_key].lteq(finish)) if finish
+      batch_relation = relation
+
+      loop do
+        if load
+          records = batch_relation.records
+          ids = records.map(&:id)
+          yielded_relation = self.where(primary_key => ids)
+          yielded_relation.load_records(records)
+        else
+          ids = batch_relation.pluck(primary_key)
+          yielded_relation = self.where(primary_key => ids)
+        end
+
+        break if ids.empty?
+
+        primary_key_offset = ids.last
+        raise ArgumentError.new("Primary key not included in the custom select clause") unless primary_key_offset
+
+        yield yielded_relation
+
+        break if ids.length < of
+        batch_relation = relation.where(arel_table[primary_key].gt(primary_key_offset))
+      end
+    end
+  end
+end