_id appended even though _id is set to false in the schema

[sebastian-nowak]

Run the following code in mongoose 5.7.5:

#!/usr/bin/env node
(async function () {
  const mongoose = require('mongoose');

  mongoose.plugin((schema) => {
    schema.options.versionKey = false;
    schema.options.minimize = false;
    schema.options.strict = 'throw';
    schema.options.strictQuery = 'throw';
  });

  mongoose.connect('mongodb://localhost:27017/test', {useNewUrlParser: true});


  const schema = new mongoose.Schema({
    _id: {
      type: mongoose.Schema.Types.ObjectId,
      default: () => mongoose.Types.ObjectId(),
      required: true
    },

    operations: {
      type: [new mongoose.Schema({
        _id: {
          type: mongoose.Schema.Types.ObjectId,
          default: () => mongoose.Types.ObjectId(),
          required: true
        },

        action: {
          type: String,
          required: true
        }
      }, {
        discriminatorKey: 'action'
      })],
      default: [],
      required: true
    }
  });

  schema.path('operations').discriminator(
    `asd/do-something`,
    new mongoose.Schema({
      wall: {
        type: mongoose.Schema.Types.ObjectId,
        required: true
      },

      pitchPath: {
        type: new mongoose.Schema({
          _id: {
            type: mongoose.Schema.Types.ObjectId,
            required: true
          },
          pitch: {
            type: mongoose.Schema.Types.ObjectId,
            required: true
          },
          path: {
            type: [{
              _id: false,
              x: {
                type: Number,
                required: true
              },
              y: {
                type: Number,
                required: true
              },
              visible: {
                type: Boolean,
                default: true,
                required: true
              }
            }],
            default: undefined,
            required: true
          }
        }),
        default: undefined,
        required: true
      }
    })
  );

  const Model = mongoose.model('test', schema);
  const instance = new Model();
  instance.operations.push({
    action: 'asd/do-something',
    wall: mongoose.Types.ObjectId(),
    pitchPath: {
      _id: mongoose.Types.ObjectId(),
      pitch: mongoose.Types.ObjectId(),
      path: [
        {x: 0, y: 0, visible: true},
        {x: 1, y: 1, visible: true}
      ]
    }
  });

  console.log(instance.operations[0].pitchPath);
})();

This is the output:

{
  _id: 5db02ad12f42a48fa5cc1d31,
  pitch: 5db02ad12f42a48fa5cc1d32,
  path: [
    { visible: true, _id: 5db02ad12f42a48fa5cc1d34, x: 0, y: 0 },
    { visible: true, _id: 5db02ad12f42a48fa5cc1d33, x: 1, y: 1 }
  ]
}

Notice that entries in the path array have ids assigned, but there's _id: false in the schema.