Find CouchDB docs missing an arbitrary field

问题:

I need a CouchDB view where I can get back all the documents that don't have an arbitrary field. This is easy to do if you know in advance what fields a document might not have. For example, this lets you send view/my_view/?key="foo" to easily retrieve docs without the "foo" field:

function (doc) {
  var fields = [ "foo", "bar", "etc" ];

  for (var idx in fields) {
    if (!doc.hasOwnProperty(fields[idx])) {
      emit(fields[idx], 1);
    }
  }
}

However, you're limited to asking about the three fields set in the view; something like view/my_view/?key="baz" won't get you anything, even if you have many docs missing that field. I need a view where it will--where I don't need to specify possible missing fields in advance. Any thoughts?

回答1:

This technique is called the Thai massage. Use it to efficiently find documents not in a view if (and only if) the view is keyed on the document id.

function(doc) {
    // _view/fields map, showing all fields of all docs
    // In principle you could emit e.g. "foo.bar.baz"
    // for nested objects. Obviously I do not.
    for (var field in doc)
        emit(field, doc._id);
}

function(keys, vals, is_rerun) {
    // _view/fields reduce; could also be the string "_count"
    return re ? sum(vals) : vals.length;
}

To find documents not having that field,

1. GET /db/_all_docs and remember all the ids
2. GET /db/_design/ex/_view/fields?reduce=false&key="some_field"
3. Compare the ids from _all_docs vs the ids from the query.

The ids in _all_docs but not in the view are those missing that field.

It sounds bad to keep the ids in memory, but you don't have to! You can use a merge sort strategy, iterating through both queries simultaneously. You start with the first id of the has list (from the view) and the first id of the full list (from _all_docs).

1. If full < has, it is missing the field, redo with the next full element
2. If full = has, it has the field, redo with the next full element
3. If full > has, redo with the next has element

Depending on your language, that might be difficult. But it is pretty easy in Javascript, for example, or other event-driven programming frameworks.

回答2:

Without knowing the possible fields in advance, the answer is easy. You must create a new view to find the missing fields. The view will scan every document, one-by-one.

To avoid disturbing your existing views and design documents, you can use a brand new design document. That way, searching for the missing fields will not impact existing views you may be already using.