# Uniqueness on has\_many associations **URL:** \ **Category:** Rails\ **Created:** [January 6, 2009, 2:03pm UTC](https://www.ruby-forum.com/t/uniqueness-on-has-many-associations/157013 "2009-01-06T14:03:03Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![tron](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/3da27b/32.png) [@tron](https://www.ruby-forum.com/u/tron)\ **Post date:** [January 6, 2009, 2:03pm UTC](https://www.ruby-forum.com/t/uniqueness-on-has-many-associations/157013/1 "2009-01-06T14:03:03Z")
Dear List, This may not be rails unique but what is the best practice to validate the uniqueness of has\_many associations. In particular the models are class Word end class PhraseComponent belongs\_to :word belongs\_to :dictionary\_entry acts\_as\_list :scope =\> :dictionary\_entry end class DictionaryEntry has\_many :phrase\_components has\_many :words, :through =\> :phrase\_components end Dictionary entries (words and phrases) are composed of words (which are uniq) and I want to check the same phrase occurs only once in the DictionaryEntry table. Is there a better or more efficient way to do this than create and update a footprint of the word associations on dictionary entries? This means I would have a string field in the dictionary\_entries table which is a concatenation of the associated words and would check the uniquness of that. If we generalize the problem, we need to assure uniqueness of lists of arbitrary objects (where a list is via a join table), in that case the only representation that I can create on the list is the concatenation of ids of list items and check the uniqueness of this? I begin to think there must be a more canonical way to handle this. Any help appreciated Viktor I found someone asking the same question but no reply followed. [http://groups.google.com/group/rubyonrails-talk/browse\_thread/thread/e4b61e90dfaa7a8/4f6e24aa5863585a?hl=en&lnk=gst&q=has\_many+uniqueness#4f6e24aa5863585a](http://groups.google.com/group/rubyonrails-talk/browse_thread/thread/e4b61e90dfaa7a8/4f6e24aa5863585a?hl=en&lnk=gst&q=has_many+uniqueness#4f6e24aa5863585a) ---
**Author:** ![tron](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/3da27b/32.png) [@tron](https://www.ruby-forum.com/u/tron)\ **Post date:** [January 7, 2009, 5:50pm UTC](https://www.ruby-forum.com/t/uniqueness-on-has-many-associations/157013/2 "2009-01-07T17:50:14Z")
Hello, I am still trying to find a solution to this. Please feel free to throw any ideas at me or help me find the appropriate forum. Thank you V ---
**Author:** ![tron](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/3da27b/32.png) [@tron](https://www.ruby-forum.com/u/tron)\ **Post date:** [January 7, 2009, 6:45pm UTC](https://www.ruby-forum.com/t/uniqueness-on-has-many-associations/157013/3 "2009-01-07T18:45:20Z")
tron wrote: > class Word > end > > class PhraseComponent > belongs\_to :word > belongs\_to :dictionary\_entry > acts\_as\_list :scope =\> :dictionary\_entry > end > > class DictionaryEntry > has\_many :phrase\_components > has\_many :words, :through =\> :phrase\_components > end Well from the database side of thing that’s easily solved by adding a unique index across the two foreign keys in the join table. That would definitely prevent duplicates in the join table. CREATE UNIQUE INDEX idx\_word\_dictionary\_entry ON phrase\_components (word\_id, dictionary\_entry\_id); From the Rails side you might be able to use something like the following example: class TeacherSchedule \< ActiveRecord::Base validates\_uniqueness\_of :teacher\_id, :scope =\> [:semester\_id, :class\_id] end Maybe (untested): class PhraseComponent \< ActiveRecord::Base validates\_uniqueness\_of :word\_id, :scope =\> :dictionary\_entry validates\_uniqueness\_of :dictionary\_entry\_id, :scope =\> :word\_id end Note: This may be an expensive method for validating uniqueness! In any of the cases mentioned you’ll still have issues related to “Optimistic Concurrency Control:” > **[Optimistic concurrency control](https://en.wikipedia.org/wiki/Optimistic_concurrency_control)** > > Optimistic concurrency control (OCC), also known as optimistic locking, is a concurrency control method applied to transactional systems such as relational database management systems and software transactional memory. OCC assumes that multiple transactions can frequently complete without interfering with each other. While running, transactions use data resources without acquiring locks on those resources. Before committing, each transaction verifies that no other transaction has modified the dat... This section of the Rails documentation has a good explanation of these issues and some options on how to solve them: [http://www.railsbrain.com/api/rails-2.2.2/doc/index.html?a=M001824&name=validates\_uniqueness\_of](http://www.railsbrain.com/api/rails-2.2.2/doc/index.html?a=M001824&name=validates_uniqueness_of) ---
**Author:** ![tron](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/3da27b/32.png) [@tron](https://www.ruby-forum.com/u/tron)\ **Post date:** [January 7, 2009, 8:58pm UTC](https://www.ruby-forum.com/t/uniqueness-on-has-many-associations/157013/4 "2009-01-07T20:58:42Z")
Hello Robert, You misunderstood my problem I guess. Think of bora as a unique word, with id 1. there is a dictionary entry for bora bora with id 2 then the phrase components representing this association are not unique in word\_id X dictionary\_id (both are 1,2) only word\_id and dictionary\_id AND list index (position) meaning there can only be one word on the same position of a list. The real problem is that I want to make sure bora bora is entered only once in the dictionary. For those who like more examples, think of travel itineraries (which are a list of cities, say New York - London - Paris - Budapest) Cities are a table, Itineraries are another and there is a join model (Visits) which acts as a list. How do I make sure an itinerary is created only once? Thanks a million all Vik On Jan 7, 5:45 pm, Robert W. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)