[Bug]: LangChain has a hardcoded value for user_column parameter

[martyna-mindsdb]

Short description of current behavior

Using the LangChain model within MindsDB as below (that is, without defining user_column and with input column being input) results in an error:

This error is mitigated by defining user_column as below:

This issue seems to be caused by the hardcoded default value for user_column here:

DEFAULT_USER_COLUMN = 'question'

But removing this line of code results in a different error.

Video or screenshots

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Expected behavior

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How to reproduce the error

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Anything else?

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