如何在Mongo中的所有文档中取消设置唯一字段？

Question

I'm using mongoose/nodeJS and I'm writing some migration scripts using node-migrate .

My update script is adding a new computed field to all documents in a collection:

exports.up = function (done) {
  Posts.find({_hash: null}, function (err, posts) {
    async.forEach(posts, function (post, next) {
      var hash = hash(post);
      post.update({_hash: hash}, next);
    }, done);
  });
};

and that works fine. I'm grabbing all docs with no hash field, computing it, and updating the record.

But node-migrate supports "downward" migrations as well, to rollback changes. In this case I want to remove the hash field from all docs that have one.

exports.down = function (done) {
  Posts.update({_hash: {$ne: null}}, {$unset: {$hash: 1}}, {multi: true}, done);
};

but i get a duplicate key error. The schema does indeed define that field as uniquely indexed, so that makes sense..?

How can I unset/delete a field from all docs in a collection if that field is defined as unique?

Thanks.

Answer 1

A unique index will not allow multiple docs where the indexed field is either missing or null .

To support this, you will need to drop the existing unique index on hash and re-create it as a unique, sparse index.

Drop the index in the shell:

db.posts.dropIndex('$_hash_1')

Update the definition of hash in your schema to make it a sparse, unique index:

hash: {type: String, index: {unique: true, sparse: true}}