Groovy / GMongo tips and tricks

January 29, 2011

Based on some emails I received last days I will show some features of GMongo that not everyone is aware.

Regex based search

In the mongo official Java driver you can do searches using Regular Expression. To do that you just need to pass a java.util.regex.Pattern instance as the search term. In Groovy there is a shortcut to create a Pattern: ~/regex/.

For example:

def pattern = ~/mypattern/
assert pattern.class.is(java.util.regex.Pattern)

Here is a small script demonstrating the usage of regex search:

// Grab the jar
@Grab('com.gmongo:gmongo:1.0')

import com.gmongo.GMongo

def gmongo = new GMongo('localhost:27017')
def db = gmongo.getDB('tips')

db.so.drop()
db.so << [name: 'Windows XP']
db.so << [name: 'Windows 7']
db.so << [name: 'Windows Vista']
db.so << [name: 'Mac OS X v10.3 "Panther"']
db.so << [name: 'Mac OS X v10.4 "Tiger"']
db.so << [name: 'Mac OS X v10.5 "Leopard"']
db.so << [name: 'Mac OS X v10.6 "Snow Leopard"']
db.so << [name: 'Mac OS X v10.7 "Lion"']

// Mac OS Only
println "\nMac Os Only\n"
db.so.find([name: ~/^Mac/]).each { so ->
    println so
}

// Print: 

// Mac OS Leopard, Snow Leopard
println "\nMac OS Leopard, Snow Leopard\n"
db.so.find([name: ~/Leopard/]).each { so ->
    println so
}

Persisting your POGO

There is an easy way to persist your POGO using GMongo. Suppose we have the class:

class Person {
   def name
   def age
}

In Groovy we can get all properties from a class using the properties property, for example:

def person = new Person(name: 'Paulo', age: 25)
println person.properties

This code outputs something like:

[
  class:class Person, 
  age:25, 
  metaClass:org.codehaus.groovy.runtime.HandleMetaClass@450a1001, 
  name:Paulo
]

As you can see, not every property means something to us. We doesn’t want to store class and metaClass in our storage, because it isn’t part of the domain. So, we need to ignore them:

println person.properties.findAll { !['class', 'metaClass'].contains(it.key) }

Now, all we need to do is persist this Map in MongoDB. Here is a complete example:

// Grab the jar
@Grab('com.gmongo:gmongo:1.0')
import com.gmongo.GMongo

class Person {
   def name
   def age
}

def gmongo = new GMongo('localhost:27017')
def db = gmongo.getDB('example')

def person = new Person(name: 'Paulo', age: 25)

db.persons.drop()
db.persons << person.properties.findAll { !['class', 'metaClass'].contains(it.key) }

def personFromMongo = db.persons.findOne()

assert 'Paulo' == personFromMongo.name
assert 25      == personFromMongo.age

BasicDBObject to POGO conversion

The class com.mongodb.BasicDBObjects extends java.util.LinkedHashMap so it’s easy to convert it to POGO, thankfully to Groovy array notation constructor call. Don’t worry, I will explain. Suppose we have the class:

class Person {
   def name
   def age
}

The two statements bellow are equivalent:

Person mike = new Person(name: 'Mike', age: 23)
Person john = [name: 'John', age: 36]

To this work you need to use typed variable. If you use untyped variable definition Groovy can’t infer what constructor to call.

So, taking advantage of this cool Groovy feature you can do things like this:

// Grab the jar
@Grab('com.gmongo:gmongo:1.0')
import com.gmongo.GMongo

class Person {
   def name
   def age
   
   def whoIAm() {
       "Hello, I'm $name and I am $age years old."
   }
}

def gmongo = new GMongo('localhost:27017')
def db = gmongo.getDB('example')

def person = new Person(name: 'Paulo', age: 25)

db.persons.drop()
db.persons << person.properties.findAll { !['class', 'metaClass'].contains(it.key) }

Person personFromMongo = db.persons.findOne().findAll { it.key != '_id' }

assert "Hello, I'm Paulo and I am 25 years old." == personFromMongo.whoIAm()

As you can see, we need to remove the _id property from the Map before instantiate the class Person, that is because Person hasn’t this property. Another alternative is add the _id property to the Person class.

Any feedback is appreciated.

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