Fixing ElasticSearch 'Unknown key for a START_ARRAY in [...].'

When you get an error message like

elasticsearch.exceptions.RequestError: RequestError(400, 'parsing_exception', 'Unknown key for a START_ARRAY in [size].')

in ElasticSearch, look for the value in the [brackets]. In our example this is [size].

Your query contains an array for that key but an array is not allowed.

For example, this query is malformed:

    "size": [10, 11]

because size takes a number, not an array.

In Python, if you are programmatically building your array, you might have an extra comma at the end of your line.

For example, if you have a line like

query["size"] = 250,

this is just syntactic sugar for

query["size"] = (250,)

i.e. it will set size to a tuple with one element. In the JSON this will result in

    "size": [250]

In order to fix that issue, remove the comma from the end of the line

query["size"] = 250

which will result in the correct query JSON

    "size": 250