eolas/Databases/MongoDB/Querying_a_collection.md

tags
Databases mongo_db node-js mongoose

Query a Mongo collection with Mongoose

We now have the following entries in our courses collection:

{
  name: 'Python Course',
  author: 'Terry Ogleton',
  tags: [ 'python', 'backend' ],
  isPublished: true,
  _id: new ObjectId("62f4e2527ac4aa2c30d41d23"),
  date: 2022-08-11T11:04:50.750Z,
  __v: 0
}
{
  name: 'Javascript Course',
  author: 'Tosh Gnomay',
  tags: [ 'js', 'frontend' ],
  isPublished: true,
  _id: new ObjectId("62f4e2527ac4aa2c30d41d24"),
  date: 2022-08-11T11:04:50.750Z,
  __v: 0
}

Now we will query the collection. This capability is provided via the Mongoose schema class we used to create the Course model. We have the following methods available to use from the schema:

• find
• findById
• findByIdAndRemove
• findByIdAndUpdate
• findOne
• findOneAndUpdate
• findOneAndRemove
• ...

The various find methods return a value that is promisified.

Return values with find

async function getCourses(){
  const courses = await Course.find()
  console.log(courses)
}

Filter values returned

This will return all the published courses where Tosh Gnomay is the author:

async function getCourses(){
  const courses = await Course.find({author: 'Tosh Gnomay', isPublished: true})
  console.log(courses)
}

This time we will filter by the same author but we will add additional parameters to distinguish:

• only the first ten entries (using .limit(10))
• sort ascending by name (using .sort({name: 1})) , to descend we would use -1)
• only return the properties name and tags for the item in the collection (using .select({name: 1, tags: 1}))

async function getCourses(){
  const courses = await Course
  .find({author: 'Tosh Gnomay', isPublished: true})
  .limit(10)
  .sort({name: 1})
  .select({name: 1, tags: 1})
  console.log(courses)
}

This returns:

[
  {
    _id: new ObjectId("62f4f07a875cff48827b8731"),
    name: 'Java Course',
    tags: [ 'java', 'backend' ]
  },
  {
    _id: new ObjectId("62f4e2527ac4aa2c30d41d24"),
    name: 'Javascript Course',
    tags: [ 'js', 'frontend' ]
  }
]

Note that the UUID is always returned, whether we specify it or not.

Filtering with operators

So far when filtering we have been doing so with reference to properties that exist on the document's model (author, isPublished etc) and we have applied tranformations on the data returned (sorting, limiting the number or matches etc.). However we can also apply operators within our queries. Operators allow us to perform computations on the data, for example: for a given numerical property on an object, return the objects for which this value is within a certain range.

When we apply operators we use the $ symbol before the operator and pass the operator function as an object.

The schema:

Model.find( { property: { $operator: conditions } } )

Comparison operators

The following comparison operators are available in MongoDB:

Operator	Function
eq	Equal to
ne	Not equal to
gt	Greater than
gte	Greater than or less than
lt	Less than
lte	Less than or equal to
in	In
nin	Not in

We can employ these comparators within a .find filter. For example let's imagine that our courses instances have a property of price.

To filter course prices that are greater than or equal to 10 and less than or equal to 29:

Course.find(({price: {$gte: 10, $lte: 20} }))

To filter course prices that are either 10, 15 or 20:

Course.find(({price: {$in: [10, 15, 20] } }))