Arel wth lateral table in the from clause, - ruby-on-rails-5

Question

I have 3 table users, roles and roles_users where in the last table is the intermediate table. So, user can have multiple roles and any role can belong to multiple users.
The problem statement is that we need to render the list of users and their roles matching the search criteria.
select users.id, dynamic_roles.name
from users, lateral (
select GROUP_CONCAT( DISTINCT( roles.name ) ) as name
from roles, roles_users
where (
( roles_users.user_id = users.id AND roles_users.role_id = roles.id )
)
) dynamic_roles
where dynamic_roles.name LIKE '%admin%' AND dynamic_roles.name LIKE '%manager%';
What I tried is as below:
rs = Role.joins(:users).select("GROUP_CONCAT( DISTINCT( #{Role.table_name}.name ) ) as name")
users = User.arel_table #predefined reference received as argument to a method that is supposed to compose the arel query.
users = users.project(users['id']).distinct
users.to_sql
=> "SELECT DISTINCT users.id FROM users"
users.from('dynamic_roles').to_sql
=> "SELECT DISTINCT users.id FROM dynamic_roles"
users.lateral('dynamic_roles').to_sql
=> TypeError: Cannot visit Arel::Nodes::Lateral
from /Users/prasadsurase/.rvm/gems/ruby-2.7.1/gems/activerecord-6.0.3.6/lib/arel/visitors/visitor.rb:39:in `rescue in visit'
Caused by NoMethodError: undefined method `visit_Arel_Nodes_Lateral' for #<Arel::Visitors::MySQL:0x00007f97424477b8>
Referring https://apidock.com/rails/v6.0.0/Arel/SelectManager/from and https://apidock.com/rails/v6.0.0/Arel/SelectManager/lateral

Answer 1

Your issue is that the mysql visitors do not include a visitor for Lateral (the way postgres does).
We could go about adding our own visitor but it is easier to get around this using a NamedFunction instead since the LATERAL syntax is the same as a function.
As with most things complicated, this is not the prettiest solution but it will provide the desired result.
roles = Arel::Table.new('roles')
role_users = Arel::Table.new('role_users')
users = Arel::Table.new('users')
group = Arel::Nodes::NamedFunction.new('GROUP_CONCAT',
[Arel::Nodes::Grouping.new(roles[:name])],
'name')
group.distinct = true
sub = roles.project(group)
.from([roles, role_users])
.where( role_users[:user_id].eq(users[:id]).and(
role_users[:role_id].eq( roles[:id]))
)
dynamic_roles = Arel::Table.new('dynamic_roles')
lateral = Arel::Nodes::NamedFunction.new('LATERAL',[sub], dynamic_roles.name)
users.project(users[:id], dynamic_roles[:name])
.from([ users, lateral])
.where( dynamic_roles[:name].matches(Arel::Nodes::Quoted.new("%admin%")).and(
dynamic_roles[:name].matches(Arel::Nodes::Quoted.new("%manager%"))
))
This will Produce the following SQL:
SELECT users.id, dynamic_roles.name
FROM users, LATERAL(
(
SELECT GROUP_CONCAT( DISTINCT (roles.name)) AS name
FROM roles, role_users
WHERE role_users.user_id = users.id AND role_users.role_id = roles.id
)
) AS dynamic_roles
WHERE
dynamic_roles.name LIKE '%admin%' AND dynamic_roles.name LIKE '%manager%'