Question 209214 in Mongodb Question posted in
The official documentation can be found here.

Conditionally update a MongoDB field based on the length of inserted values

I want to update a field in MongoDB only if the string value being inserted has a longer length (number of characters) than what is currently stored in the database. How can I achieve this?

Here is my current python code:

from pymongo import MongoClient, UpdateOne

# connect to client
...

query = {'jk': 'some_key_for_query'} # lookup in collection
d = {'s': 'abcd'} # field to update
bulk_update.append(UpdateOne(query, {'$set': d}, upsert=True))
# update
d = {'s': 'abcdefg'}
bulk_update.append(UpdateOne(query, {'$set': d}, upsert=True))
# don't update
d = {'s': 'ab'}
bulk_update.append(UpdateOne(query, {'$set': d}, upsert=True))

# bulk insert
...

During the first update, the field is set to ‘abcd’. In the second update, I expect the update to occur because the value ‘abcdefg’ has more characters than the current value in the database. However, I don’t want the third update to take place because the value ‘ab’ is shorter. How can I modify my code to achieve this behavior?

Tags:

2

Answers


  1. 0

    I guess this will solve your problem:

    values_to_update = ['abcd', 'abcdefg', 'ab']

for new_val in values_to_update:
    doc = collection.find_one({'jk': 'some_key_for_query'})

    # if the document doesn't exist or new_val is longer than the existing value, perform the update
    if not doc or len(new_val) > len(doc.get('s', '')):
        collection.update_one({'jk': 'some_key_for_query'}, {'$set': {'s': new_val}}, upsert=True)
    Login or Signup to reply.
  2. 0

    Since you are using upsert, one option is to use update with pipeline, which allows to use the current values in the query:

    bulk_update.append(UpdateOne(query, 
  [{$set: {
      s: {$cond: [
          {$gte: [{$strLenCP: {$ifNull: ["$s", ""]}}, new_str_length]},
          "$s",
          new_string
      ]}
  }}],
  upsert=True))

    See how it works on the playground example

    Login or Signup to reply.
Please signup or login to give your own answer.

Back To Top
Search