mysql - Getting empty migration when II try and create a unique index in my rails project -

i’m using rails 4.2.3 mysql 5.5.37. want create unique constraint on table, involving multiple columns, ran …

rails generate migration add_index :user_objects, [:day, :object, :user], :unique => true 

however, produces file, db/migrate/20160203003708_add_index.rb, had nothing in …

class addindex < activerecord::migration   def change   end end 

so nothing happens when run “rake db:migrate”. i’m doing wrong in attempt create unique index across multiple columns , what’s right way on command line?

according migrations guide , migrations api docs generating migrations command line supports creating new tables , adding new columns. table needs created or column(s) added determined name of migration using part after _to_. e.g. running command like:

rails generate migration add_username_to_user_objects username:string:uniq 

would generate migration adds unique usernamecolumn (even if exists) user_objects table. now, if command supported adding indexes etc. syntax hard remember , fragile (because of bash etc) many prefer writing code file anyway :) :

# db/migrate/20160203003708_add_index.rb def change     add_index :user_objects, [:day, :object, :user], unique: true end 

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a suggestion: order of fields in unique index affects performance , other rules regarding sql. e.g. if user fk users table, putting first cover queries use user in where, order, group by etc (e.g. select ... user_objects user_id = ...) without needing separate index it. more important factor cardinality, "how unique each of day, object, userin user_objects?" put unique first.