Calculate position based on a value

just a casual observer... however..

the only place in your query where there is a ? would be "AND a1.from_user_id=?".

have you included the parameter substitution, like you did before (i.e., $this->db->where('from_user_id', $user_id);

OM...

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And on the pedestal these words appear:
'My name is Ozymandias, king of kings:
Look on my works, ye Mighty, and despair!'
Nothing beside remains. Round the decay
Of that colossal wreck, boundless and bare
The lone and level sands stretch far away.