Notes

Mongo Commands

5 min read

db.runCommand({command: 1}) used to run some commands like listCollections to list collections that are on the db

  • db: used to show current database
  • show dbs: show all existing databases
  • use db, switch or create new
  • db.dropDatabase(“df”) to delete

Collections

  • db.createCollection(“dd”)
  • db.collection.insert() creates during insertion
  • db.collection.drop()
  • there is also db.collection.renameCollection()
  • db.getCollectionNames()

Querying

db.users.find({}, {}) is used to do queries on collections, the first parenthesis is used for query conditions, the second one is used for projections

  • $all is like contains all of what I AM SPECIFYING and is not exact equal$in is contains at least one
  • grades: [10], this is the exact equalt

Regex

  • Start a regex pattern with /pattern/, ^ indicates the word will start with it, $ indicates the word will end with it.
  • .* means allow any character in between
  • \b\w+\b matches words
  • \b means word barrier
  • \w is caracther
  • \s is a space character

Projection

Projection is done in the second parenthesis, writing {field: 0} means all except this field, 1 means print this field, using 0 and 1 together in a single query is not allowed. You can also add a field by writing just the field name and value other than 1 or 0 to it, even if this field exists in the document, ex: db.users.find({}, {name: 1, message: "hello"}), this add the name field from the document and writes a new field named message that does not exist in the document.

NOTE: if the id_ is not explicitly suppressed in the projection parenthesis it will always be printed

db.users.find().sort({age: 1, name: 1}) will first sort by age then name, 1 means sort by ascending order. To sort by descending order use -1

Logical operations

  • To negate a query use $not: {} before the query function that we are going to use or as the call it a query document
  • To use a logical operations like “and” or “or” to get fields, start the query parameter with it, for ex: db.users.find({$and: [{age: 10}, {age: 12}}) also db.users.find({age: 10, grade: 20}) is an implicit and.
  • Only and, nor, or are top level starters the others are called field level operators
  • $not is not a top level function like AND and OR, you use it to negate an a expression example: {name: $not: {$eq: "Ahmed"}}, and its parameter is a function or a regex. $ne is the same for $not: {$eq}

NOTE: Implicit ANDs are a bit tricky, if we do {age: 12, age: 17} the database will only consider the last one, but it an implicit and with function operations is like {$age: {$gte: 15, $lte 17}} here we are looking for ages more than 15 and less than 17

Important Functions

-Doing db.students.find({grades: {$size: 2}}) prints the student that has 2 grades in it array, while writing db.students.find({}, {grades: {$size: "$grades"}) Will write the size of the grades array in the collection.

NOTE: We put $ before fields in our collection in places where we can create fields, like in projection.

Mathematical operations in mongo

Aggregation

The output of the current stage is the input of the next stage

NOTE: to make a complex group, make the group a an object like _id: {name: $as, t: $smt}

  • to output into another collection use the pipeline {$out: collection name}
  • to merge into a collection use {$merge: {into: "name", on: "_id", whenMatched: "replace", whenNotMatched: "insert"}}
  • $count is a pipeline

Geospatial Queries

To operate on data that is in 3d space, create a field that has the following fields:

{
	type: <Geospatial object type>,
	coordinates: []
}
// You can instalntly put the coordinats and the default will be point

To do queries on this data with geospatial function an index must be added with the coordinate types: db.students.createIndex({"name": "2dsphere" for earth like sphere coordinates or "2d" for 2d space}) $geoWithin does not require index, but it will improve performance

// geoWithin takes a a shape opertotor or geometry function
{
$geoWithin:
	$center: [[], degree] or polygon or $geometry: {type: , coordinates}
		same with geoINtersects or nearSphere: [cooeds], $maxDistance
}

nearSphere → distance geoWithin → degrees

Deletion

db.collections.drop()deletes the whole collection and the indices, .remove({}) deletes the documents and can take a query argument. There is no such thing as delete(). Note that in remove we will have to brackets even if they are empty, else it is an error.

NOTE: There is deleteOne and deleteMany though

Updating

  • db.students.update({}, {}) updates only the first entry, use updateMany to update all, there is no function as updateAll

Indices

  • Indices reduce the complexity of the search
  • I can’t have more than two text indices in the collection
  • to create text indices use db.collection.createIndex({field1: “text”, field2: “text”}), they cannot be created seperately, Ill get an error
  • to drop it do db.collection.dropIndex(“field1_text”)
  • there is also getIndexes
  • to search text use {$text: {$search: "dsd"}}

From slides

u can also renameCollection and getCollectionNames

I can put {out, query, finalize} in the end

Graph View