Mongoose Relationships Finding a Parent From the Child

Learning objective: By the end of this lesson, students will be able to find a parent document based on specific properties of a child document using Mongoose.

Finding a parent from a child

Imagine you need to find a parent document, but your starting point is a property held within one of its child documents. Thankfully, Mongoose query methods are sophisticated enough to filter results based on the properties of subdocuments.

Let’s see an example using the findOne() method:

const todo = await Todo.findOne({
  'subtasks.text': 'Learn how props work'
})

Notice the syntax used. We specify a property (text) of an object within the subtasks array, and the value we’re searching for. This operation returns the parent todo that contains a subtask that matches this criteria.

With access to the parent todo, we can now use a JavaScript array method like find() to access the appropriate subtask:

const todo = await Todo.findOne({
  'subtasks.text': 'Learn how props work'
})

const subtask = todo.subtasks.find((subtask) => {
  return subtask.text === 'Learn how props work'
})

Real-world scenario

Consider an application where a user can see a list of all the subtasks. The user wants to delete a specific subtask, but each document has been removed from the context of its parent. To achieve this functionality, we need to first find the correct parent document, and then find and remove the appropriate subtask within.

Let’s build out the relevant pieces now. Add the following to queries.js at the bottom of the Query Functions section:

const findParentAndRemoveSubtask = async () => {
  const foundTodo = await Todo.findOne({
    'subtasks.text': 'Learn how props work'
  });

  const foundSubtask = foundTodo.subtasks.find((subtask) => {
    return subtask.text === 'Learn how props work'
  });

  foundSubtask.deleteOne();

  await foundTodo.save();
  console.log('Updated todo:', foundTodo);
};

💡 The deleteOne() method is accessible on all mongoose subdocuments.

Next, call findParentAndRemoveSubtask in the runQueries function:

const runQueries = async () => {
  console.log('Queries running.');
  await findParentAndRemoveSubtask();
};

Finally, run the queries.js file with the following command:

node queries.js

You should see that the relevant subtask has been removed from the subtasks array.