fix slow query for product users

app/controllers/api/v1/products/relationships/users_controller.rb

@@ -12,15 +12,15 @@ class UsersController < Api::V1::BaseController
     authorize :product
 
     def index
-      users = apply_pagination(authorized_scope(apply_scopes(product.users)).preload(:role))
+      users = apply_pagination(authorized_scope(apply_scopes(product_users)).preload(:role))
       authorize! users,
         with: Products::UserPolicy
 
       render jsonapi: users
     end
 
     def show
-      user = FindByAliasService.call(product.users, id: params[:id], aliases: :email)
+      user = FindByAliasService.call(product_users, id: params[:id], aliases: :email)
       authorize! user,
         with: Products::UserPolicy
 
@@ -31,6 +31,10 @@ def show
 
     attr_reader :product
 
+    # FIXME(ezekg) Uses a more optimized query for large accounts. This should
+    #              be considered a bug in union_of.
+    def product_users = current_account.users.for_product(product)
+
     def set_product
       scoped_products = authorized_scope(current_account.products)
 

app/models/user.rb

@@ -309,7 +309,27 @@ def owned = where(owner: proxy_association.owner)
     end
   }
 
-  scope :for_product, -> id { joins(:licenses).where(licenses: { product_id: id }).distinct }
+  scope :for_product, -> id {
+    license_users = LicenseUser.arel_table
+    licenses      = License.arel_table
+    users         = User.arel_table
+
+    # More optimized union query for this particular association
+    left_outer_joins(:license_users)
+      .joins(
+        Arel::Nodes::InnerJoin.new(
+          licenses,
+          Arel::Nodes::On.new(
+            licenses[:user_id].eq(users[:id]).or(
+              licenses[:id].eq(license_users[:license_id]),
+            )
+          ),
+        ),
+      )
+      .where(
+        licenses: { product_id: id },
+      )
+  }
   scope :for_license, -> id { joins(:licenses).where(licenses: { id: id }).distinct }
   scope :for_group_owner, -> id { joins(group: :owners).where(group: { group_owners: { user_id: id } }).distinct }
   scope :for_user, -> id {