CODEWITHSUNDEEP
mongodb
meso_2600
meso_2600

Reputation: 2126

Mongodb find_and_update $ne issue

I have a document:

active -> array of active id's

new_active -> array of active id's with some details

Now, I would like to add a new ID to active, but only if it's not present in new_active. I dont want to change my schema, I would rather like to understand why first version of the code works perfectly fine, while second one does not.

In the first code I have used arrays for both active & new_active, and this works fine: 

a = MongoMapper.database.collection(:papers).find_and_modify(
        :query => {
          :active => {
            '$nin' => [paper_id]
          },
          :new_active =>  {'$ne' => [paper_id]} #I know, $nin can also be used
        },
        :update => {
          '$push' =>{
            :active => paper_id,
          }
        },
        :upsert => false,
        :new => true

This code works fine and pushes ID to the active if it is not present in both arrays. Now, I wanted to have an array of hashes/ dicts in new_active. This is when problem occurred. The reason for using $ne instead of $nin is that $ne should return a result it the field doesn't have particular value OR the field doesn't exist. In this case, paper_id field in the new_active doesn't exist: 

a = MongoMapper.database.collection(:papers).find_and_modify(
        :query => {
          :active => {
            '$nin' => [paper_id]
          },
          :new_active => {
            :paper_id => {'$ne' => [paper_id]}
          }
        },
        :update => {
          '$push' =>{
            :active => paper_id,
          }
        },
        :upsert => false,
        :new => true

This code doesnt push paper_id into the active array. Why?

Upvotes: 0

Views: 95

Answers (1)

rakslice
rakslice

Reputation: 8975

The condition {'$nin' => [paper_id]} is equivalent to {'$ne' => paper_id}, not {'$ne' => [paper_id]}.

Upvotes: 1

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