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We have a custom field - 80% of our contacts have this custom field in their profile - the rest are blank.
We setup a rule - if 7 characters or more of the custom filed are a match - it meets the threshold - however - contacts with blank custom fields are coming up as possible duplicates. Why is that?
Latest Civi - Drupal 7.
It's coming from line 179 in CRM/Dedupe/BAO/Rule.php, but it looks as if it's been like this forever so I'm not sure if there's a reason.
If you change line 76 where it sets $on to be the below code instead, it makes it work but I can't immediately see if that has any side-effects for other types of rules.
$on = [
"SUBSTR(t1.{$this->rule_field}, 1, {$this->rule_length}) = SUBSTR(t2.{$this->rule_field}, 1, {$this->rule_length})",
"t1.{$this->rule_field} IS NOT NULL",
"t1.{$this->rule_field} <> ''",
];
Note that dedupe checking caches results, so after you make that change clear cache at admin - system settings - cleanup caches.
