Build triple UNION query using Arel (Rails 5)

user has_many tasks

I'm trying to create a 3 select UNION:

Task.from("(#{select1.to_sql} UNION #{select2.to_sql} UNION #{select3.to_sql}) AS tasks")

But with Arel.

I can easily use Arel for UNION query for 2 selects:

tasks_t = Task.arel_table
select1 = tasks_t.project('id').where(tasks_t[:id].eq(1)) # SELECT id FROM tasks WHERE tasks.id = 1
select2 = Task.joins(:user).select(:id).where(users: {id: 15}).arel  # SELECT "tasks".id FROM "tasks" INNER JOIN "users" ON "users"."id" = "tasks"."user_id" WHERE "users"."id" = 15
union = select1.union(select2) # SELECT * FROM users WHERE users.id = 1 UNION SELECT * FROM users WHERE users.id = 2

union_with_table_alias = tasks_t.create_table_alias(union, tasks_t.name)
Task.from(union_with_table_alias)
# SELECT "tasks".* FROM ( SELECT id FROM "tasks" WHERE "tasks"."id" = 1 UNION SELECT "tasks"."id" FROM "tasks" INNER JOIN "users" ON "users"."id" = "tasks"."user_id" WHERE "users"."id" = $1 ) "tasks"  [["id", 15]]

#=> Task::ActiveRecord_Relation [#<Task: id: 1>, #<Task: id: 2>]

How can I do it with triple union select?

select3 = tasks_t.project('id').where(tasks_t[:id].eq(3)) # SELECT id FROM tasks WHERE tasks.id = 3

Something like:

triple_union = select1.union(select2).union(select3)
triple_union_with_table_alias = tasks_t.create_table_alias(triple_union, tasks_t.name)
Task.from(triple_union_with_table_alias)

Which should roughly translate to

Task.from("(#{select1.to_sql} UNION #{select2.to_sql} UNION #{select3.to_sql}) AS tasks")

Note the above line also fails: Caused by PG::ProtocolViolation: ERROR: bind message supplies 0 parameters, but prepared statement "" requires 1

To summarize: How to use Arel to build a tripple UNION query? select 1 UNION select 2 UNION select 3

Thank you

Ruby 2.5.3 Rails 5.2.4 Arel 9.0.0

You can not invoke union in an Arel::Nodes::Union, because in the method definition the ast of both objects is called. And the result of a union operation doesn't respond to ast:

def union(operation, other = nil)
  if other
    node_class = Nodes.const_get("Union#{operation.to_s.capitalize}")
  else
    other = operation
    node_class = Nodes::Union
  end

  node_class.new(self.ast, other.ast)
end

What you can do is to manually call Arel::Nodes::Union, passing as any of those arguments the result of a union:

Arel::Nodes::Union.new(Arel::Nodes::Union.new(select1, select2), select3)