Rails Confused with refrential integrity - ruby-on-rails

Question

The more i read about foreign keys in rails i am getting more and more confused. In a post i read that its sufficient to add belongs_to and has_many/has_one in respective model file to getting things done. But again in another post I read that the index should be added to reference another table. Suppose There is writers table and book table in dbms while creating Books table we have to add
FOREIGN KEY (writers_Id) REFERENCES Writers(Id)
but in rails we in writer model we add has_many :book an in book model we add belongs_to :writer is both are equivalent ?
If both are equivalent then why we add index such as
add_index :books, :writer_id
I have project on which I am working on it has users has one personal information, academic information, application and rank. Also there is subject_streams which have streams and streams have cutoffs. Finally there is category which is independent. I dont know if i modeled data correctly but in the schema.rb am I doing correctly what i have said ?
ActiveRecord::Schema.define(version: 20140713133617) do
create_table "academics", force: true do |t|
t.integer "user_id"
t.integer "tenth_roll", default: 0
t.integer "tenth_year_pass", default: 2000
t.decimal "tenth_marks_percent", precision: 10, scale: 2, default: 40.0
t.string "tenth_board"
t.integer "hs_roll", default: 0
t.integer "hs_year_pass", default: 2002
t.decimal "hs_marks_percent", precision: 10, scale: 2, default: 40.0
t.string "hs_board"
t.datetime "created_at"
t.datetime "updated_at"
t.integer "calculated_marks"
t.string "sub1"
t.integer "sub1_marks"
t.string "sub2"
t.integer "sub2_marks"
t.string "sub3"
t.integer "sub3_marks"
t.string "sub4"
t.integer "sub4_marks"
t.string "sub5"
t.integer "sub5_marks"
t.string "sub6"
t.integer "sub6_marks"
t.string "sub7"
t.integer "sub7_marks"
t.string "sub8"
t.integer "sub8_marks"
t.string "sub9"
t.integer "sub9_marks"
t.string "sub10"
t.integer "sub10_marks"
t.integer "subject_streams_id"
end
add_index "academics", ["user_id"], name: "index_academics_on_user_id", unique: true, using: :btree
create_table "applications", force: true do |t|
t.integer "user_id"
t.integer "stream_id"
t.datetime "created_at"
t.datetime "updated_at"
t.boolean "verified", default: false
end
add_index "applications", ["user_id"], name: "index_applications_on_user_id", unique: true, using: :btree
create_table "categories", force: true do |t|
t.string "category"
t.datetime "created_at"
t.datetime "updated_at"
end
create_table "cutoffs", force: true do |t|
t.integer "stream_id"
t.integer "category_id"
t.datetime "created_at"
t.datetime "updated_at"
t.integer "cutoff_marks"
end
add_index "cutoffs", ["stream_id"], name: "index_cutoffs_on_stream_id", using: :btree
create_table "personals", force: true do |t|
t.integer "user_id"
t.date "date_of_birth"
t.string "gender"
t.string "blood_group"
t.string "fathers_name"
t.string "mothers_name"
t.text "address_present"
t.datetime "created_at"
t.datetime "updated_at"
t.string "first_name"
t.string "middle_name"
t.string "last_name"
t.integer "category_id"
t.string "image"
t.string "avatar"
end
add_index "personals", ["user_id"], name: "index_personals_on_user_id", unique: true, using: :btree
create_table "ranks", force: true do |t|
t.integer "user_id"
t.integer "rank"
t.datetime "created_at"
t.datetime "updated_at"
end
add_index "ranks", ["user_id"], name: "index_ranks_on_user_id", unique: true, using: :btree
create_table "registers", force: true do |t|
t.boolean "active"
t.datetime "created_at"
t.datetime "updated_at"
end
create_table "simple_captcha_data", force: true do |t|
t.string "key", limit: 40
t.string "value", limit: 6
t.datetime "created_at"
t.datetime "updated_at"
end
add_index "simple_captcha_data", ["key"], name: "idx_key", using: :btree
create_table "streams", force: true do |t|
t.string "stream"
t.datetime "created_at"
t.datetime "updated_at"
t.integer "seats"
t.integer "subject_stream_id"
end
add_index "streams", ["subject_stream_id"], name: "index_streams_on_subject_stream_id", using: :btree
create_table "subject_streams", force: true do |t|
t.string "subject_stream"
t.datetime "created_at"
t.datetime "updated_at"
end
create_table "users", force: true do |t|
t.string "email", default: "", null: false
t.string "encrypted_password", default: "", null: false
t.string "reset_password_token"
t.datetime "reset_password_sent_at"
t.datetime "remember_created_at"
t.integer "sign_in_count", default: 0, null: false
t.datetime "current_sign_in_at"
t.datetime "last_sign_in_at"
t.string "current_sign_in_ip"
t.string "last_sign_in_ip"
t.datetime "created_at"
t.datetime "updated_at"
t.integer "level", default: 1
end
add_index "users", ["email"], name: "index_users_on_email", unique: true, using: :btree
add_index "users", ["reset_password_token"], name: "index_users_on_reset_password_token", unique: true, using: :btree
end

Answer 1

In Ruby on Rails, the code has_many :books and belongs_to :writer is like defining relationships in the code level. It doesnt achieve anything in the database level. For example, if we write belongs_to :writer in the Book model, it just means that if there is a column writer_id in the books table, we can code something like this:
b = Book.first
b_writer = b.writer
#### equivalent to
# b_writer = Writer.find(b.writer_id)
# b_writer = Writer.where(:id => b.writer_id).first
# or b_writer = Writer.find_by_sql("SELECT writers.* from writers where writers.id = #{b.writer_id}")
It assumes that the foreign key is writer_id in books table by convention. If we need to change the foreign key for the queries generated by association, we need to specify it separately:
belongs_to :writer, :foreign_key => 'writerID'
So, in general declaring associations in models, gives us some utility methods to query those associations from database, instead of creating the queries manually. So, all the database related changes needs to be done in the migrations, like adding the column writer_id, adding index for column writer_id, setting writer_id with forieign key constraint etc. Rails does not support foreign_key constraint in migrations by default, as different databases handle foreign_key differently. For PostGre and MySQL database, you can use foreigner gem for adding foreign_key constraint.
Please read more about Rails associations