MongoDB: The Complete Guide/

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Read Documents: Part 4

Read Documents: Part 4

Learn and practice commands like find, findOne with elements operator and evaluation operators into the filter query.

We'll cover the following...

Element operators

$exists operator

The $exists operator is used to check the existence of a field and return documents.

Let’s insert some documents to use with the$exists operator query.

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db.tasks.insertMany([
{
      name: 'Task 1',
	    priority: 1,
},
{
      name: 'Task 2',
      status: 'pending',
}
]);

Next, we build a query to return a document that has a field value set to priority.

db.tasks.find({
    priority: {
        $exist: true
    }
});

This query returns the below output.

[
  {
    _id: ObjectId("60fa599e384ee438a9eb2f96"),
    name: 'Task 1',
    priority: 1
  }
]

It doesn’t return Task 2.

Let’s build a query to return a document that does not have a priority field value.

db.tasks.find({
    priority: {
        $exist: false
    }
});

This query returns the below output.

[
  {
    _id: ObjectId("60fa599e384ee438a9eb2f97"),
    name: 'Task 2',
    status: 'pending'
  }
]

It doesn’t return Task 1.

$type operator

We use the $type operator to check the type of the field, and return documents if a match is found.

Let’s insert some documents.

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db.tasks.insertMany([
  {
      name: 'Task 1',
      due_date: new Date(),
      priority: 1,
      categories: [
      {
        key: 'category_a'
      },
      {
        key: 'category_b'
      }
     ]
  }
]);

Next, we ...