[ http://opensource.atlassian.com/projects/hibernate/browse/HHH-772?page=co... ] Kai Kunstmann commented on HHH-772: ----------------------------------- After long thinking about it, I realize now how this "feature request" of allowing null-values in a map is hard to implement reasonably. I'm writing here to help other people understand the problem and maybe introduce another idea for a solution: It is true, that a {{java.util.Map}} is defined in terms of a {{java.util.Set}} of {{java.util.Map.Entry}} with no entry having a key equal to any other entry's key in the same entry-set, and that people want to persist every entry of that set no matter if its missing a value, i.e. they want to persist the _absence of a value_ while not giving up the relation between key and map-owner -- especially with a {{<map-key-many-to-many>}}. There doubtlessly are use-cases for this, and it truly is a reasonable request -- if you look at it from the Java point-of-view. I would like to do that, too, instead of coding complicated workarounds. However, with a simple {{<one-to-many>}} mapping in place of the map-value (even so with a {{<composite-element>}}) it becomes impossible to express the desired _absence of value_, since the key would have to go into the same row as the missing entity (or component), which means having an empty row with nothing in it -- especially no id -- but a key and a reference to the owner to express the _absence of an entity_. This is obviously absurd, and in my opinion the only true reason why this issue _won't fix_. With a {{<one-to-many>}} mapping the value of a map-entry essentially _is_ the entry, which is likewise questionable. A solution to this predicament could be the introduction of an {{<idmap>}} with a surrogate {{<collection-id>}}, which obviously cannot contain a {{<one-to-many>}} mapping but could be made capable of expressing _absence of value_ by the requirement of {{table}} attribute and a {{<many-to-many>}}, {{<many-to-any>}}, {{<element>}} or {{<composite-element>}} mapping as with the {{<idbag>}} to serve as a standalone entry-row with an explicit id. There certainly are other considerations to make -- it's just an idea... -- This message is automatically generated by JIRA. - If you think it was sent incorrectly contact one of the administrators: http://opensource.atlassian.com/projects/hibernate/secure/Administrators.... - For more information on JIRA, see: http://www.atlassian.com/software/jira