fix(update): cast right hand side of $pull as a query instead of an…

… update for document arrays

Fix #8166

lib/helpers/query/castUpdate.js

@@ -4,6 +4,7 @@ const CastError = require('../../error/cast');
 const StrictModeError = require('../../error/strict');
 const ValidationError = require('../../error/validation');
 const castNumber = require('../../cast/number');
+const cast = require('../../cast');
 const getEmbeddedDiscriminatorPath = require('./getEmbeddedDiscriminatorPath');
 const handleImmutable = require('./handleImmutable');
 const utils = require('../../utils');
@@ -139,6 +140,17 @@ function walkUpdatePath(schema, obj, op, options, context, filter, pref) {
     key = keys[i];
     val = obj[key];
 
+    // `$pull` is special because we need to cast the RHS as a query, not as
+    // an update.
+    if (op === '$pull') {
+      schematype = schema._getSchema(prefix + key);
+      if (schematype != null && schematype.schema != null) {
+        obj[key] = cast(schematype.schema, obj[key], options, context);
+        hasKeys = true;
+        continue;
+      }
+    }
+
     if (val && val.constructor.name === 'Object') {
       // watch for embedded doc schemas
       schematype = schema._getSchema(prefix + key);