Getting going quickly with Python, MongoDB and Spatial data

jieforest

We can also grab pieces of the requested URL and pass it into the function. By enclosing a part of the URL in a < >, it indicates that we want to access it within our function. Here you can see where if the url looks like:

http://www.mysite.com/name/steve

Then the reponse will be Hello, steve!

Or the URL could be http://www.mysite.com/name

Hello, Stranger!

We are going to define URL mappings for some basic REST like functionality to interact with our spatial MongoDB data store.

jieforest

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jieforest

Modify the source code

The first function we are going to write will be to just simply return all the records in the database. In a more full featured app you would probably want to add pagination and other features to this query but we won’t be doing that today.

1. @app.route(“/ws/parks”)
   
2. def parks():
   
3. #setup the connection
   
4. conn = pymongo.Connection(os.environ[‘OPENSHIFT_NOSQL_DB_URL’])
   
5. db = conn.parks
   
6. #query the DB for all the parkpoints
   
7. result = db.parkpoints.find()
   

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1. #Now turn the results into valid JSON
   
2.     return str(json.dumps({'results':list(result)},default=json_util.default))
   

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I chose to put the web services under the url /ws/parks so that we could use other parts of the URL namespace for other functionality. You can now go to your application URL (http://pythonws-.rhcloud.com/ws/parks) and you should be able to see all the documents in the DB.

jieforest

Using MongoDB in Python

In the code above we simply make a connection to the MongoDB instance for this application and then execute a query. The pymongopackage provides all the functionality to interact with the MongoDB instance from our Python code.

The pymongo commands are very similar to the MongoDB command line interaction except two word commands like db.collection.findOne are split with a _, such as db.collection.find_one. Please go to the pymongo site to read more about the documentation.

Notice we use the environment variables to specify the connection URL. While not hard coding database connection parameters is good practice in non-cloud apps, in our case you MUST use the environment variables.

Since your app can be idled and then spun up or it could be autoscaled, the IP and ports are not always guaranteed. By using the environment variables we make our code portable.

We pass the result set (which comes back as a Python dictionary) into json.dump so we can return JSON straight to the client. Since pymongo is returning the results in UTF and we want just plain text, we need to pass the json_util.default from the bson library into the json.dump command.

This is probably the easiest experience I have ever had writing a web service. I love Flask, Pymongo, and Python for the simplicity of “Just Getting Stuff Done”.

jieforest

Grab just one park

Next we will implement the code to get back a park given a parks uniqueID. For ID we will just use the ID generated by MongoDB on document insertion (_id). The ID looks like a long random sequence and that is what we will pass into the URL.

return a specific park given it’s mongo _id

1. @app.route(“/ws/parks/park/”) def onePark(parkId): #setup the connection conn = pymongo.Connection(os.environ[‘OPENSHIFT_NOSQL_DB_URL’]) db = conn.parks
   
2.     #query based on the objectid
   
3.     result = db.parkpoints.find({'_id': objectid.ObjectId(parkId)})
   
4. 
   
5.     #turn the results into valid JSON
   
6.     return str(json.dumps({'results' : list(result)},default=json_util.default))
   

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Here you have to use another class from the bson library - ObjectID. The actual ObjectID in MongoDB is an object and so we have to take the ID passed in on the url and create an Object from it. The ObjectID class allows us to create one of these objects to pass into the query. Other than that the code is the same as above.

This little snippet also shows an example of grabbing part of the URL and passing it to a function. I explained this concept above but here we can see it in practice.