Elasticsearch error - [script] unknown field [source], parser not found

Issue

If you’re using a script for Elasticsearch, e.g. a Function Score query, you may spot [script] unknown field [source], parser not found error after upgrading Elasticsearch to 5.6.0 or higher and the library for your back-end language. In my case ruflin/elastica that resulted in mentioned error on Travis.

How did I solve it?

I didn’t notice that Travis by default installs older Elasticsearch 5.2 rather than the latest 5.x because there was no other issue before. In older versions there was no source field used now by default by the upgraded Elastica library, there was inline that got deprecated.

Basically all that was needed was upgrading Elasticsearch on Travis, you can add e.g.:

before_install:
  - curl -O https://artifacts.elastic.co/downloads/elasticsearch/elasticsearch-5.6.10.deb && sudo dpkg -i --force-confnew elasticsearch-5.6.10.deb && sudo service elasticsearch restart

to .travis.yml for 5.6.10 version.