projects: [
  {
    type: Schema.Types.ObjectId,
    ref: 'Project'
  }
]

const client = await Client.updateOne(
  { _id: req.body.client_id },
  {
    $pullAll: { projects: [req.body.project_id] }
  }
);

const client = await Client.updateOne(
  { _id: req.body.client_id },
  {
    $pullAll: { projects: req.body.project_id }
  }
);

const client = await Client.updateOne(
  { _id: req.body.client_id },
  {
    $pullAll: { projects: { $in: [ req.body.project_id ] } }
  }
);