SELECT source_merchant.id, target_merchant.id, COUNT(target_merchant.id) /
((
    (SELECT COUNT(*) FROM tagged WHERE model = 'Merchant' AND model_id = source_merchant.id) +
    (SELECT COUNT(*) FROM tagged WHERE model = 'Merchant' AND model_id = target_merchant.id)
) /2 ) as similarity
FROM merchants source_merchant
LEFT JOIN tagged source_merchant_tags ON (
    source_merchant.id = source_merchant_tags.model_id AND
    source_merchant_tags.model = 'Merchant'
)
INNER JOIN tagged target_merchant_tags ON (
    source_merchant_tags.tag_id = target_merchant_tags.tag_id
    AND (source_merchant_tags.model = 'Merchant' AND target_merchant_tags.model = 'Merchant')
    AND (source_merchant_tags.model_id != target_merchant_tags.model_id)
)
LEFT JOIN merchants target_merchant ON (
    target_merchant_tags.model_id = target_merchant.id AND target_merchant_tags.model = 'Merchant'
)
WHERE source_merchant.id = 2
GROUP BY source_merchant.id, target_merchant.id
ORDER BY similarity DESC
LIMIT 5

public function related_merchants() {
    return $this->hasManyThroug(relations_stuff_i_cannot_imagine...)
        ->selectRaw("SELECT source_merchant.id, target_merchant.id, COUNT(target_merchant.id) /
        ((
            (SELECT COUNT(*) FROM tagged WHERE model = 'Merchant' AND model_id = source_merchant.id) +
            (SELECT COUNT(*) FROM tagged WHERE model = 'Merchant' AND model_id = target_merchant.id)
        ) /2 ) as similarity")
        ->groupBy('source_merchant.id', 'target_merchant.id ')
        ->orderBy('similarity')
        ->limit(5);
}