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View tables

ActiveRecord::Base.connection.tables

Delete any table.

    ActiveRecord::Base.connection.drop_table("users")
    ------------OR----------------------
    ActiveRecord::Migration.drop_table(:users)
    ------------OR---------------------
    ActiveRecord::Base.connection.execute("drop table users")

Remove index from existing column

 ActiveRecord::Migration.remove_index(:users, :name => 'index_users_on_country')

where country is a column name in the migration file with already added index in users table as shown below:-

 t.string :country,add_index: true

Remove foreign key constraint

ActiveRecord::Base.connection.remove_foreign_key('food_items', 'menus')

where menus has_many food_items and their respective migrations too.

Add column

ActiveRecord::Migration.remove_column :table_name, :column_name

for example:-

 ActiveRecord::Migration.add_column :profiles, :profile_likes, :integer, :default => 0

Any method in Rails model can return boolean value.

simple method-

  ##this method return ActiveRecord::Relation
  def check_if_user_profile_is_complete
    User.includes( :profile_pictures,:address,:contact_detail).where("user.id = ?",self)
  end

Again simple method returning boolean value-

  ##this method return Boolean(NOTE THE !! signs before result)
  def check_if_user_profile_is_complete
    !!User.includes( :profile_pictures,:address,:contact_detail).where("user.id = ?",self)
  end

So,the same method will now return boolean instead of anything else :).

Sometimes we want to use a where query on a a collection of records returned which is not ActiveRecord::Relation.Hence we get the above error as Where clause is know to ActiveRecord and not to Array.

There is a precise solution for this by using Joins.

EXAMPLE:-

Suppose i need to find all user profiles(UserProfile) which are active which is not a user(User) with an id=10.

UserProfiles.includes(:user=>:profile_pictures]).where(:active=>true).map(&:user).where.not(:id=>10)

So above query will fail after map as map will return an array which will not work with where clause.

But using joins,will make it work,

UserProfiles.includes(:user=>:profile_pictures]).where(:active=>true).joins(:user).where.not(:id=>10)

As joins will output similar records like map but they will be ActiveRecord and not an Array.