Build triple UNION query using Arel (Rails 5) - ruby-on-rails

Question

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

Answer 1

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)