Finding What You're Looking For

Finding What You're Looking For

A previous blog entry looked at simple searching with find(). Now we will look deeper into building queries. This blog covers:

• Numbers and searching ranges
• Arrays and searching in them
• Building logical expressions
• Regular expressions for super flexible searching

Numbers and Ranges

Numeric data is inserted into MongoDB without quotes, just as we'd treat a number in any programming language:

db.eg.insert({Name : "Bill", Age : 18})

db.eg.insert({Name : "Ted", Age : 17})

and we can search for exact numbers:

db.eg.find({Age:17})

or those in a range:

db.eg.find({Age: {$lt : 18}})

Other useful operators include:

• $lte - Less than or equal
• $gt - Greater than
• $gte - Greater than or equal
• $ne - Not equal

Inserting and Searching Arrays

To search for each and any of an array of possible matches on the same field, you can specify an array of target values. This is better than a string of $or operators.

db.eg.find({Name : { $in : ["Bill" , "Ted"]}})

Note the [ ] square brackets to  denote the array.

You can insert an array as a value in a document:

db.eg.insert({Likes: ["Ice cream","Apples","Chocolate"]})

and search for one or more items in it:

db.eg.find({Likes: { $in: ["Apples","Gin"]}})

If you want to match a single term to the array, the single term still needs to be in an array of length one:

db.eg.find({Likes: { $in: ["Apples"]}})

The opposite of $in is $nin, which means not in.

Building Logical Expressions

I've already hinted at it: you can build logical expressions for a search using $and, $or, $not and $nor. A simple AND is implemented implicitly by MongoDB if you provide a comma separated list of matches:

db.eg.find({Name: "Bill", Age: 18})

is the same as

db.eg.find({$and: [{Name: "Bill"}, {Age: 18}]})

but we must be explicit if we want to use OR:

db.eg.find({$or: [{Name: "Bill"}, {Age: 17}]})

$Not does as you'd expect and $nor means not and not.

Finally, if you want to specify more complex matches on a single value, you can use regular expressions, which are described below:

Searching with Regular Expressions

You can build some reasonably complex searches with a mixture of AND, OR, and NOT, but logical expressions are less useful if you want to match certain classes of string. For example, a search to find all the names that include a number (Like Joe90 or Ben10). For this, we need regular expressions.

There are two ways to tell MongoDB you want to use a regular expression. One is to use the Javascript notation and the other is to use the MongoDB $regex operator.

The Javascript notation puts the regular expression between slashes, followed by any options. Here is the syntax:

/expression/options

and the $regex operator makes things explicit:

{ $regex: 'expression', $options: 'options' }

And now some examples from a collection called db.colc:

find({Name: /ev/})	Find names that contain "ev" anywhere
find({Name: /^K/i})	Find names that start with "K", ignoring case (K or k)
find({Name: /[abc]/})	Find names that contain "a" or "b" or "c" anywhere
find({Name: /\w{1,}\d{1,}/})	Find names that end with some numbers

It all looks a bit cryptic, but you get the idea - you can specify pretty much any pattern you want. To do it confidently off the top of your head takes a bit of practice, but its worthwhile as regular expressions have many other uses. Once you can do it in Javascript, for example, you can validate web forms before they are submitted.

A bit more information on regular expressions in MongoDB can be found here. For more details on the Javascript method of defining regular expressions, look here.

First Steps in MongoDB

First Steps in mongoDB

In this article, I'm going to use the mongo shell to play with some simple DB queries. See the post on installation if you don't yet have mongo installed. Here is what I'll be doing today

• Starting a new collection to store some simple documents
• Putting some documents into the collection
• Listing the contents of the collection
• Searching the collection for documents with certain values (e.g Male)
• Searching the collection for documents with certain keys (e.g.Gender)
• Removing a document from the collection

Later posts will cover more complex searching using regular expressions and tasks such as listing all keys in a collection.

Get a Collection

A collection is somewhere to put a set of related documents. The relationship is loose - not like a relational database, just things that belong together. You can create one like this:

db.creatCollection(colname)

where colname is the name you want to give your new collection, and you refer to existing collections as:

db.colname

For example,

db.createCollection("books")

and you refer to the collection as

db.books

Note the " quotes around the creation (it is a string here) but not around the reference, where it is a database object. Also, you do not have to create the collection before using it.

Not sure what collections your database contains? No problem:

show collections

will tell you.

You can just insert data into a collection that doesn't exist, and one will be created. How to do that is next:

Insert a Document

No surprises here, you insert a document using

db.books.insert(doc)

where doc is defined as a set of key: value pairs like this:

doc={ k1:v1, k2:v2 ... }

for example,

db.books.insert({Name:"On the Road", Author: "Jack Kerouac"})

Or, if we first say

b= db.books

then we can just use

b.insert (.....)

as a shorter way to reference the collection.

Perhaps the next book doesn't have an author, but an editor, so:

db.books.insert({Name:"Collected Poems", Editor: "J.E Bowles"})

Notice, we haven't defined a table structure - we just add documents in this style and new keys are added as needed.

List the Contents of a Collection

The find() command is used to retrieve data from a MongoDB:

b.find()

returns every entry in the collection. We will learn to iterate over rows later, for now just see that this produces:

> db.books.find()
{ "_id" : ObjectId("52658e0a84b47fef69ebab5f"), "Name" : "On the Road", "Author" : "Jack Kerouac" }
{ "_id" : ObjectId("52658e4984b47fef69ebab60"), "Name" : "Collected Poems", "Editor" : "J.E Bowles" }

Note the _id given to each document in the collection - MongoDB assigns a unique ID to each one as they are entered.

Search for Certain Values

Searches are defined in much the same way as data is entered, by specifying key: value pairs:

b.find({Name : "Collected Poems"})

Searches for the book with the Name of "Collected Poems". We will look at more complex searches in another blog.

Search for Certain Key

Perhaps you want to know all the books with an editor, but don't know any of the editors' names. You can search for documents with a certain key like this:

db.books.find( { Editor:{ "$exists":"true"}})

This is our first peek at the $, which prefixes a set of MongoDB operators, described here. We will look at them in a future blog.

Removing a Document

To remove a document from a collection, use

db.books.remove({Editor:"J.E. Bowles"})

Note that the syntax is the same as for find (except the word remove replaces the word find, of course).

Well, that's it for now. Next stop, more detailed searching.