Fixes error on discriminator models if unique set on the base model (…

…mongoose-unique-validator#85)

index.js

@@ -100,6 +100,12 @@ module.exports = function(schema, options) {
                             } else if (isFunc(this.model)) {
                                 model = this.model(this.constructor.modelName);
                             }
+                            // Is this model a discriminator and the unique index is on the whole collection,
+                            // not just the instances of the discriminator? If so, use the base model to query.
+                            // https://github.com/Automattic/mongoose/issues/4965
+                            if (model.baseModelName && indexOptions.partialFilterExpression == null) {
+                              model = model.db.model(model.baseModelName);
+                            }
 
                             model.where({ $and: conditions }).countDocuments((err, count) => {
                                 resolve(count === 0);