Only add_metadata if no matching value

[johnjamesjacoby]

add_metadata() has a $unique parameter, but it only looks for a unique meta_key for the given object ID. I have a need to add meta-data with the same meta_key but different values, and only if that meta_value isn't already present with the same key.

To do this now, requires:

• Getting all meta-data for the ID by meta_key
• Checking those array-values for my value
• Bailing if it's already present, or...
• Proceeding with $unique set to false

It'd be lovely if add_metadata() either:

• Accepted strings like key, value for the $unique parameter, to switch up the check
• Added a second parameter for unique value in addition to a unique key

[johnjamesjacoby]

Proof of concept. Needs testing with serialized data.

[johnjamesjacoby]

Thinking about this more, there could be 4 types of uniqueness:

• Unique meta_key to all objects
• Unique meta_key to the $object_id (what $unique currently checks)
• Unique meta_key and meta_value to all objects
• Unique meta_key and meta_value the $object_id (what my patch adds)