[WIP] Refactor tenant enforcement to use Arel visitor pattern

[a5-stable]

citusdata/activerecord-multi-tenant#223

• Tried to fix the issue where delete_all and update_all would turn into subqueries when using LIMIT.
• The tenant condition was missing from the outer query.

Before

UPDATE "projects" SET "name" = "new"
WHERE "projects"."id" IN (
  SELECT "projects"."id" FROM "projects"
  WHERE "projects"."account_id" = 1 LIMIT 5
)

After

UPDATE "projects" SET "name" = "new"
WHERE "projects"."id" IN (
  SELECT "projects"."id" FROM "projects"
  WHERE "projects"."account_id" = 1 LIMIT 5
) AND "projects"."account_id" = 1

However, Subqueries are added even when they are unnecessary.

Example

Projects.update_all
# => UPDATE "projects" SET "name" = "new"
#    WHERE "projects"."id" IN (
#      SELECT "projects"."id" FROM "projects"
#      WHERE "projects"."account_id" = 1
#    ) AND "projects"."account_id" = 1

Investigation

• The tenant condition does not appear in the outer query because:
  • Inside visit_Arel_Nodes_DeleteStatement, the wheres are replaced with a subquery in prepare_delete_statement(https://github.com/rails/rails/blob/d8795bd762ab2716b6e7fd07a403566b28159cdc/activerecord/lib/arel/visitors/to_sql.rb#L22).
  • Any tenant conditions added before this end up inside the subquery.
• Possible solution: replace prepare_delete_statement.