PyMongo Monday – Episode 3 – Read

20180927_173519

Previously we covered:
* Episode 1: Setting Up Your MongoDB Environment
* Episode 2: CRUD – Create

In this episode (episode 3) we are are going to cover the Read part of CRUD. MongoDB provides a query interface through the find function.We are going to demonstrate Read by doing find queries on a collection
hosted in MongoDB Atlas. The MongoDB
connection string is:

mongodb+srv://demo:demo@demodata-rgl39.mongodb.net/test?retryWrites=true

This is a cluster running a database called demo with a single collection called zipcodes. Every ZIP code in the US is in this database. To connect to this cluster we are going to use the Python shell.

$ cd ep003
$ pipenv shell
Launching subshell in virtual environment…
JD10Gen:ep003 jdrumgoole$  . /Users/jdrumgoole/.local/share/virtualenvs/ep003-blzuFbED/bin/activate
(ep003-blzuFbED) JD10Gen:ep003 jdrumgoole$ python
Python 3.6.5 (v3.6.5:f59c0932b4, Mar 28 2018, 03:03:55)
[GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>>
>>> from pymongo import MongoClient
>>> client = MongoClient(host="mongodb+srv://demo:demo@demodata-rgl39.mongodb.net/test?retryWrites=true")
>>> db = client["demo"]
>>> zipcodes=db["zipcodes"]
>>> zipcodes.find_one()
{'_id': '01069', 'city': 'PALMER', 'loc': [-72.328785, 42.176233], 'pop': 9778, 'state': 'MA'}
>>>

The find_one query will get the first record in the collection. You can see
the structure of the fields in the returned document. The _id is the ZIP code.
The city is the city name. The loc is the GPS coordinates of each ZIP code. The pop is the population size and the state is the two letter state code.
We are connecting with the default user demo with the password demo. This
user only has read-only access to this database and collection.

So what if we want to select all the ZIP codes for a particular city?

Querying in MongoDB consists of constructing a partial JSON document that matches the fields you want to select on. So to get all the ZIP codes in the city of PALMER we use the following query

>>> zipcodes.find({'city': 'PALMER'})

>>>

Note we are using find() rather than find_one() as we want to return
all the matching documents. In this case find() will return a
cursor.

To print the cursor contents just keep calling .next() on the cursor
as follows:

>>> cursor=zipcodes.find({'city': 'PALMER'})
>>> cursor.next()
{'_id': '01069', 'city': 'PALMER', 'loc': [-72.328785, 42.176233], 'pop': 9778, 'state': 'MA'}
>>> cursor.next()
{'_id': '37365', 'city': 'PALMER', 'loc': [-85.564272, 35.374062], 'pop': 1685, 'state': 'TN'}
>>> cursor.next()
{'_id': '50571', 'city': 'PALMER', 'loc': [-94.543155, 42.641871], 'pop': 1119, 'state': 'IA'}
>>> cursor.next()
{'_id': '66962', 'city': 'PALMER', 'loc': [-97.112214, 39.619165], 'pop': 276, 'state': 'KS'}
>>> cursor.next()
{'_id': '68864', 'city': 'PALMER', 'loc': [-98.241146, 41.178757], 'pop': 1142, 'state': 'NE'}
>>> cursor.next()
{'_id': '75152', 'city': 'PALMER', 'loc': [-96.679429, 32.438714], 'pop': 2605, 'state': 'TX'}
>>> cursor.next()
Traceback (most recent call last):
  File "", line 1, in 
  File "/Users/jdrumgoole/.local/share/virtualenvs/ep003-blzuFbED/lib/python3.6/site-packages/pymongo/cursor.py", line 1197, in next
    raise StopIteration
StopIteration

As you can see cursors follow the Python iterator protocol and will raise a StopIteration exception when the cursor is exhausted.

However, calling .next() continously is a bit of a drag. Instead you can
import the pymongo_shell package and call the print_cursor() function. It will print out twenty records at a time.

>>> from pymongo_shell import print_cursor
>>> print_cursor(zipcodes.find({'city': 'PALMER'}))
{'_id': '01069', 'city': 'PALMER', 'loc': [-72.328785, 42.176233], 'pop': 9778, 'state': 'MA'}
{'_id': '37365', 'city': 'PALMER', 'loc': [-85.564272, 35.374062], 'pop': 1685, 'state': 'TN'}
{'_id': '50571', 'city': 'PALMER', 'loc': [-94.543155, 42.641871], 'pop': 1119, 'state': 'IA'}
{'_id': '66962', 'city': 'PALMER', 'loc': [-97.112214, 39.619165], 'pop': 276, 'state': 'KS'}
{'_id': '68864', 'city': 'PALMER', 'loc': [-98.241146, 41.178757], 'pop': 1142, 'state': 'NE'}
{'_id': '75152', 'city': 'PALMER', 'loc': [-96.679429, 32.438714], 'pop': 2605, 'state': 'TX'}
>>>

If we don’t need all the fields in the doc we can use
projection to remove some fields. This is a second doc argument to the find() function. This doc can specify the fields to return explicitly.

>>> print_cursor(zipcodes.find({'city': 'PALMER'}, {'city':1,'pop':1}))
{'_id': '01069', 'city': 'PALMER', 'pop': 9778}
{'_id': '37365', 'city': 'PALMER', 'pop': 1685}
{'_id': '50571', 'city': 'PALMER', 'pop': 1119}
{'_id': '66962', 'city': 'PALMER', 'pop': 276}
{'_id': '68864', 'city': 'PALMER', 'pop': 1142}
{'_id': '75152', 'city': 'PALMER', 'pop': 2605}

To include multiple fields in a query just add them to query doc. Each field is
treated as a boolean and to select the documents that will be returned.

>>> print_cursor(zipcodes.find({'city': 'PALMER', 'state': 'MA'}, {'city':1,'pop':1}))
{'_id': '01069', 'city': 'PALMER', 'pop': 9778}
>>>

To pick documents with one field or the other we can use the $or operator.

>>> print_cursor(zipcodes.find({ '$or' : [ {'city': 'PALMER' }, {'state': 'MA'}]}))
{'_id': '01069', 'city': 'PALMER', 'loc': [-72.328785, 42.176233], 'pop': 9778, 'state': 'MA'}
{'_id': '01002', 'city': 'CUSHMAN', 'loc': [-72.51565, 42.377017], 'pop': 36963, 'state': 'MA'}
{'_id': '01012', 'city': 'CHESTERFIELD', 'loc': [-72.833309, 42.38167], 'pop': 177, 'state': 'MA'}
{'_id': '01073', 'city': 'SOUTHAMPTON', 'loc': [-72.719381, 42.224697], 'pop': 4478, 'state': 'MA'}
{'_id': '01096', 'city': 'WILLIAMSBURG', 'loc': [-72.777989, 42.408522], 'pop': 2295, 'state': 'MA'}
{'_id': '01262', 'city': 'STOCKBRIDGE', 'loc': [-73.322263, 42.30104], 'pop': 2200, 'state': 'MA'}
{'_id': '01240', 'city': 'LENOX', 'loc': [-73.271322, 42.364241], 'pop': 5001, 'state': 'MA'}
{'_id': '01370', 'city': 'SHELBURNE FALLS', 'loc': [-72.739059, 42.602203], 'pop': 4525, 'state': 'MA'}
{'_id': '01340', 'city': 'COLRAIN', 'loc': [-72.726508, 42.67905], 'pop': 2050, 'state': 'MA'}
{'_id': '01462', 'city': 'LUNENBURG', 'loc': [-71.726642, 42.58843], 'pop': 9117, 'state': 'MA'}
{'_id': '01473', 'city': 'WESTMINSTER', 'loc': [-71.909599, 42.548319], 'pop': 6191, 'state': 'MA'}
{'_id': '01510', 'city': 'CLINTON', 'loc': [-71.682847, 42.418147], 'pop': 13269, 'state': 'MA'}
{'_id': '01569', 'city': 'UXBRIDGE', 'loc': [-71.632869, 42.074426], 'pop': 10364, 'state': 'MA'}
{'_id': '01775', 'city': 'STOW', 'loc': [-71.515019, 42.430785], 'pop': 5328, 'state': 'MA'}
{'_id': '01835', 'city': 'BRADFORD', 'loc': [-71.08549, 42.758597], 'pop': 12078, 'state': 'MA'}
{'_id': '01845', 'city': 'NORTH ANDOVER', 'loc': [-71.109004, 42.682583], 'pop': 22792, 'state': 'MA'}
{'_id': '01851', 'city': 'LOWELL', 'loc': [-71.332882, 42.631548], 'pop': 28154, 'state': 'MA'}
{'_id': '01867', 'city': 'READING', 'loc': [-71.109021, 42.527986], 'pop': 22539, 'state': 'MA'}
{'_id': '01906', 'city': 'SAUGUS', 'loc': [-71.011093, 42.463344], 'pop': 25487, 'state': 'MA'}
{'_id': '01929', 'city': 'ESSEX', 'loc': [-70.782794, 42.628629], 'pop': 3260, 'state': 'MA'}
Hit Return to continue

We can do range selections by using the $lt and $gtoperators.

>>> print_cursor(zipcodes.find({'pop' : { '$lt':8, '$gt':5}}))
{'_id': '05901', 'city': 'AVERILL', 'loc': [-71.700268, 44.992304], 'pop': 7, 'state': 'VT'}
{'_id': '12874', 'city': 'SILVER BAY', 'loc': [-73.507062, 43.697804], 'pop': 7, 'state': 'NY'}
{'_id': '32830', 'city': 'LAKE BUENA VISTA', 'loc': [-81.519034, 28.369378], 'pop': 6, 'state': 'FL'}
{'_id': '59058', 'city': 'MOSBY', 'loc': [-107.789149, 46.900453], 'pop': 7, 'state': 'MT'}
{'_id': '59242', 'city': 'HOMESTEAD', 'loc': [-104.591805, 48.429616], 'pop': 7, 'state': 'MT'}
{'_id': '71630', 'city': 'ARKANSAS CITY', 'loc': [-91.232529, 33.614328], 'pop': 7, 'state': 'AR'}
{'_id': '82224', 'city': 'LOST SPRINGS', 'loc': [-104.920901, 42.729835], 'pop': 6, 'state': 'WY'}
{'_id': '88412', 'city': 'BUEYEROS', 'loc': [-103.666894, 36.013541], 'pop': 7, 'state': 'NM'}
{'_id': '95552', 'city': 'MAD RIVER', 'loc': [-123.413994, 40.352352], 'pop': 6, 'state': 'CA'}
{'_id': '99653', 'city': 'PORT ALSWORTH', 'loc': [-154.433803, 60.636416], 'pop': 7, 'state': 'AK'}
>>>

Again sets of $lt and $gt are combined as a boolean and. if you need different logic you can use the boolean operators.

Conclusion

Today we have seen how to query documents using a query template, how to reduce the output using projections and how to create more complex queries using boolean and $lt and $gt operators.

Next time we will talk about the Update portion of CRUD.

MongoDB has a very rich and full featured query language including support
for querying using full-text, geo-spatial coordinates and nested queries.
Give the query language a spin with the Python shell using the tools we
outlined above. The complete zip codes data set is publicly available for read
queries at the MongoDB URI:

mongodb+srv://demo:demo@demodata-rgl39.mongodb.net/test?retryWrites=true”

Try MongoDB Atlas via the free-tier
today. A free MongoDB cluster for your own personal use forever!

PyMongo Monday Episode 2 : Create

Last time we showed you how to setup up your environment.

In the next few episodes we will take you through the standard CRUD operators that every database is expected to support. In this episode we will focus on the Create in CRUD.

Create

Lets look at how we insert JSON documents into MongoDB.

First lets start a local single instance of mongod using m.

$ m use stable
2018-08-28T14:58:06.674+0100 I CONTROL [main] Automatically disabling TLS 1.0, to force-enable TLS 1.0 specify --sslDisabledProtocols 'none'
2018-08-28T14:58:06.689+0100 I CONTROL [initandlisten] MongoDB starting : pid=43658 port=27017 dbpath=/data/db 64-bit host=JD10Gen.local
2018-08-28T14:58:06.689+0100 I CONTROL [initandlisten] db version v4.0.2
2018-08-28T14:58:06.689+0100 I CONTROL [initandlisten] git version: fc1573ba18aee42f97a3bb13b67af7d837826b47
2018-08-28T14:58:06.689+0100 I CONTROL [initandlisten] allocator: system

etc...

The mongod starts listening on port 27017 by default. As every MongoDB driver
defaults to connecting on localhost:27017 we won’t need to specify a connection string explicitly in these early examples.

Now, we want to work with the Python driver. These examples are using Python
3.6.5 but everything should work with versions as old as Python 2.7 without problems.

Unlike SQL databases, databases and collections in MongoDB only have to be named to be created. As we will see later this is a lazy creation process, and the database and corresponding collection are actually only created when a document is inserted.

$ python
Python 3.6.5 (v3.6.5:f59c0932b4, Mar 28 2018, 03:03:55)
[GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>>
>>> import pymongo
>>> client = pymongo.MongoClient()
>>> database = client[ "ep002" ]
>>> people_collection = database[ "people_collection" ]
>>> result=people_collection.insert_one({"name" : "Joe Drumgoole"})
>>> result.inserted_id
ObjectId('5b7d297cc718bc133212aa94')
>>> result.acknowledged
True
>>> people_collection.find_one()
{'_id': ObjectId('5b62e6f8c3b498fbfdc1c20c'), 'name': 'Joe Drumgoole'}
True
>>>

First we import the pymongo library (line 6). Then we create the local client proxy object,
client = pymongo.MongoClient() (line 7) . The client object manages a connection pool to the server and can be used to set many operational parameters related to server connections.

We can leave the parameter list to the MongoClient call blank. Remember, the server by default listens on port 27017 and the client by default attempts to connect to localhost:27017.

Once we have a client object, we can now create a database, ep002 (line 8)
and a collection, people_collection (line 9). Note that we do not need an explicit DDL statement.

Using Compass to examine the database server

A database is effectively a container for collections. A collection provides a container for documents. Neither the database nor the collection will be created on the server until you actually insert a document. If you check the server by connecting MongoDB Compass you will see that there are no databases or collections on this server before the insert_one call.

screen shot of compass at start

These commands are lazily evaluated. So, until we actually insert a document into the collection, nothing happens on the server.

Once we insert a document:

>>>> result=database.people_collection.insert_one({"name" : "Joe Drumgoole"})
>>> result.inserted_id
ObjectId('5b7d297cc718bc133212aa94')
>>> result.acknowledged
True
>>> people_collection.find_one()
{'_id': ObjectId('5b62e6f8c3b498fbfdc1c20c'), 'name': 'Joe Drumgoole'}
True
>>>

We will see that the database, the collection, and the document are created.

screen shot of compass with collection

And we can see the document in the database.

screen shot of compass with document

_id Field

Every object that is inserted into a MongoDB database gets an automatically
generated _id field. This field is guaranteed to be unique for every document
inserted into the collection. This unique property is enforced as the _id field
is automatically indexed
and the index is unique.

The value of the _id field is defined as follows:

ObjectID

The _id field is generated on the client and you can see the PyMongo generation code in the objectid.py file. Just search for the def _generate string. All MongoDB drivers generate _id fields on the client side. The _id field allows us to insert the same JSON object many times and allow each one to be uniquely identified. The _id field even gives a temporal ordering and you can get this from an ObjectID via the generation_time method.

>>> from bson import ObjectId
>>> x=ObjectId('5b7d297cc718bc133212aa94')
>>> x.generation_time
datetime.datetime(2018, 8, 22, 9, 14, 36, tzinfo=)
>>> print(x.generation_time)
2018-08-22 09:14:36+00:00
>>>

Wrap Up

That is create in MongoDB. We started a mongod instance, created a MongoClient proxy, created a database and a collection and finally made then spring to life by inserting a document.

Next up we will talk more abou Read part of CRUD. In MongoDB this is the find query which we saw a little bit of earlier on in this episode.


For direct feedback please pose your questions on twitter/jdrumgoole that way everyone can see the answers.

The best way to try out MongoDB is via MongoDB Atlas our Database as a Service.
It’s free to get started with MongoDB Atlas so give it a try today.