Validation for uniqueness not working - ruby-on-rails

Question

I have a model called as Template which has self referential has-many through association with a join model called as RelatedTemplate.
The Template model is written like this:
has_many :related_templates, :foreign_key => :parent_id
has_many :children, :through => :related_templates, :source => :child
has_many :inverse_related_templates, class_name: "RelatedTemplate", :foreign_key => :child_id
has_many :parents, :through => :inverse_related_templates, :source => :parent
validates :name, presence: true, uniqueness: { case_sensitive: false },
length: { maximum: 100, too_long: "should be maximum %{count} characters" }
The join-model RelatedTemplate is:
belongs_to :parent, class_name: 'Template'
belongs_to :child, class_name: 'Template'
In the console when i tried to create a child template with the same name as the parent name then the validation doesn't work:
rails c --sandbox
Loading development environment in sandbox (Rails 4.2.5)
Any modifications you make will be rolled back on exit
irb(main):001:0> a = Template.new
=> #<Template id: nil, client_id: nil, something_id: nil, name: nil, description: nil, another_something_id: nil, video_id: nil, container_id: nil>
irb(main):002:0> a.something_id=1
=> 1
irb(main):003:0> a.name="Hamza"
=> "Hamza"
irb(main):004:0> a.another_something_id=16
=> 16
irb(main):005:0> a.container_id=2
=> 2
irb(main):006:0> a.children.build(name: "Hamza", another_something_id: 16, container_id: 2)
=> #<Template id: nil, client_id: nil, something_id: nil, name: "Hamza", description: nil, another_something_id: 16, video_id: nil, container_id: 2>
irb(main):007:0> a.save
(0.9ms) SAVEPOINT active_record_1
Template Exists (1.3ms) SELECT 1 AS one FROM "templates" WHERE LOWER("templates"."name") = LOWER('Hamza') LIMIT 1
Container Load (0.7ms) SELECT "containers".* FROM "containers" WHERE "containers"."id" = $1 LIMIT 1 [["id", 2]]
Template Exists (0.5ms) SELECT 1 AS one FROM "templates" WHERE LOWER("templates"."name") = LOWER('Hamza') LIMIT 1
Container Load (0.2ms) SELECT "containers".* FROM "containers" WHERE "containers"."id" = $1 LIMIT 1 [["id", 2]]
SQL (0.3ms) INSERT INTO "templates" ("something_id", "name", "another_something_id", "container_id") VALUES ($1, $2, $3, $4) RETURNING "id" [["something_id", 1], ["name", "Hamza"], ["another_something_id", 16], ["container_id", 2]]
SQL (0.2ms) INSERT INTO "templates" ("name", "another_something_id", "container_id") VALUES ($1, $2, $3) RETURNING "id" [["name", "Hamza"], ["another_something_id", 16], ["container_id", 2]]
SQL (0.6ms) INSERT INTO "related_templates" ("parent_id", "child_id", "created_at", "updated_at") VALUES ($1, $2, $3, $4) RETURNING "id" [["parent_id", 156], ["child_id", 157], ["created_at", "2016-05-05 07:03:51.496346"], ["updated_at", "2016-05-05 07:03:51.496346"]]
(0.2ms) RELEASE SAVEPOINT active_record_1
=> true
Now the query:
irb(main):016:0> a.name
=> "Hamza"
irb(main):017:0> a.children.first.name
Template Load (1.9ms) SELECT "templates".* FROM "templates" INNER JOIN "related_templates" ON "templates"."id" = "related_templates"."child_id" WHERE "related_templates"."parent_id" = $1 ORDER BY "templates"."id" ASC LIMIT 1 [["parent_id", 158]]
=> "Hamza"
Dont know what is wrong here ?

Answer 1

Both object only exist in memory, when you validate them. The uniqueness validation passes in both cases, because the validation checks if there is a similar record in the database already.
Have a look at the logs:
# you call save
irb(main):007:0> a.save
(0.9ms) SAVEPOINT active_record_1
# Rails validates the first object
Template Exists (1.3ms) SELECT 1 AS one FROM "templates" WHERE LOWER("templates"."name") = LOWER('Hamza') LIMIT 1
Container Load (0.7ms) SELECT "containers".* FROM "containers" WHERE "containers"."id" = $1 LIMIT 1 [["id", 2]]
# Rails validates the second object
Template Exists (0.5ms) SELECT 1 AS one FROM "templates" WHERE LOWER("templates"."name") = LOWER('Hamza') LIMIT 1
Container Load (0.2ms) SELECT "containers".* FROM "containers" WHERE "containers"."id" = $1 LIMIT 1 [["id", 2]]
# At this point Rails thinks that both records will be fine, because the validation passed
# Rails saved the first object
SQL (0.3ms) INSERT INTO "templates" ("something_id", "name", "another_something_id", "container_id") VALUES ($1, $2, $3, $4) RETURNING "id" [["something_id", 1], ["name", "Hamza"], ["another_something_id", 16], ["container_id", 2]]
# Rails saved the second object
SQL (0.2ms) INSERT INTO "templates" ("name", "another_something_id", "container_id") VALUES ($1, $2, $3) RETURNING "id" [["name", "Hamza"], ["another_something_id", 16], ["container_id", 2]]
This is good example why Rails uniqueness validations are not 100% secure. Another example might be race conditions, when multiple requests hit the application at the same time.
The only way to avoid this kind of problems, is to add a unique index to the database table.