mongo

MongoDB

• Database vs collections
• Documents (records) = one entry in the collections
• MongoDB creates IDs for documents automatically

Queries

Find

• dp.pets.find({ type: "dog" }).limit(5).toArray()
  • find returns a cursor to it on
  • toArray returns an array
• dp.pets.count({ type: "cat", age: { $gt: 12 } }) , $gt is a query operator
• dp.pets.count({ type: "bird", $and: [ {age: {$gte: 4}}, {age: {$lte: 8}} ] })
• dp.pets.find({ type: "dog" }).sort({ age: -1, name: -1 }), -1 indicates descending.
• dp.pets.find({ type: "dog" }, { name: 1, _id: -1 }), projection, use 1 or true and -1 or false to include/exclude fields.

Update

Use One if only updating one.

dp.pets.updateOne({
    { type: "dog", ... },  // query object
    { $set: { owner: "Brain Holt" } }
})
 
dp.pets.updateMany({
    { type: "dog" },  // query object
    { $inc: { age: 1 } }
})
 
dp.pets.updateMany({
    { type: "dog" },  // query object
    { $set: { ... } },
    { upsert: true }  // insert if not exist, using the $set object
})

Delete

• deleteOne
• findOneAndDelete, similar to pop

Indexes

• A B-tree is created, complexity reduced from $O(n)$ to $O(\lg n)$
• ...find(...).explain("executionStats") to view performance details
• db.pets.createIndex( { name: 1 } )
• db.pet.getIndexies()
• db.pet.createIndex( {index: 1}, {unique: true} ) to create unique indices.
• db.pets.createIndex({type: "text", breed: "text", name: "text"}), only one text index is allowed. db.pets.find({ $text: {$search: "dog Havanese Luna"} }), which can be sorted by textScore. Full text search.

Aggregation

db.pets.aggregate([
    { $match : { type: "dog" } },  // first stage in pipeline
    {
        $bucket: {
            groupBy: $age",
            boundaries: [0, 3, 9, 15],  // bottom included, top excluded
            default: "16+",
            output: { count: { $sum: 1 } }  // similar to projection
        }
    }, 
    { $sort: { count: -1 } } // final stage
])

Ops

• Replica set
• Primary - read and write to
• Secondaries - transactions in primary are flushed to secondaries, “eventually consistent”
• When primary goes down, an election is made to choose new primary.