code: function assertInstance(instance, model) {
    const modelName = get('modelName')(model)
    try {
      mongoose.model(modelName)
    } catch (err) {
      throw Error(`no such model as ${modelName}`)
    }
    const errMsg = stripIndents`
      Expected an instance of ${modelName} but got this:
      ${JSON.stringify(instance, undefined, 2)}`
    assert(instance instanceof model, errMsg)
  }
docs: Throws if a given object is not an instance of given model.
  @method assertInstance
  @param {Mixed} instance - Could be anything, but most likely a mongoose Model
  @param {Model} model - The Model from Mongoose (see mongoose.model('User'))
  @return {undefined}
  @throws {Error} - Throws if input is not an instance of model, or if model doesn't exist