CastError: Cast to string failed for value “{}“ at path “xxx“ for model “xxxModel“

1. 问题

在执行mongodb查询语句时报错CastError: Cast to string failed for value "{}" at path "xxx" for model "xxxModel"

var where = { key: { '$in': {} } };
var data = await mdbModel.find(where).exec();

2. 解析

遇到这种强制转换错误的，往往时查询的语句的问题或者就是查询语句语法问题

3. 修复

查询某个参数中的 $in 只能接受数组参数，而不是对象，所以查询就报错了。

The $in operator selects the documents where the value of a field equals any value in the specified array. To specify an $in expression, use the following prototype:

{ field: { $in: [<value1>, <value2>, ... <valueN> ] } }

正确代码:

var where = { key: { '$in': [] } };
var data = await mdbModel.find(where).exec();