Instructions for querying Publisher's database

The instructions on the GOV.UK developer docs don't work for querying
 data in Publisher's database. Add some instructions for how to query
 Publisher's database in deployed environments.

README.md

@@ -57,6 +57,10 @@ bundle exec rake state_machines:draw CLASS=Edition TARGET=docs
 
 This will generate a diagram in the `docs/state_machines` folder.
 
+### Querying the database of a deployed publisher app
+
+Publisher stores its data in DocumentDB, which can't be queried using the instructions detailed in the GOV.UK developer docs. Instead, follow [these instructions for querying the database](docs/database-querying.md). 
+
 ## Further documentation
 
 - [Fact Checking](docs/fact-checking.md)

docs/database-querying.md

@@ -0,0 +1,38 @@
+## Querying the database
+
+Publisher uses MongoDB for its data storage (technically, it only uses MongoDB when running locally—on AWS it uses DocumentDB, which is "Mongo-compatible"). There are two ways of querying the data stored in the database:
+1. using [Mongoid](https://www.mongodb.com/docs/mongoid/current/)
+2. using a Mongo shell.
+
+The GOV.UK developer docs document [how to open a database command-line session](https://docs.publishing.service.gov.uk/manual/databases.html#open-a-database-commmand-line-session) of a Rails application deployed to k8s, but this doesn't work for Publisher, as attempting to run the specified command results in an exception.
+
+To query the database, first [establish access to the k8s cluster](https://docs.publishing.service.gov.uk/kubernetes/get-started/access-eks-cluster/#access-a-cluster-that-you-have-accessed-before), then [open a rails console](https://docs.publishing.service.gov.uk/kubernetes/cheatsheet.html#open-a-rails-console) on one of the publisher pods.
+
+### Querying using Mongoid
+
+Mongoid is "the officially supported object-document mapper (ODM) for MongoDB in Ruby". It can be used to query the database using Rails models.
+
+From within the Rails console, query the data using [Mongoid](https://www.mongodb.com/docs/mongoid/current/reference/queries/). For example:
+
+```shell
+Edition.first
+=> #<GuideEdition _id: ...>
+```
+returns the first edition in the database, as a Rails model object.
+
+### Querying using a Mongo shell
+
+One thing to be aware of with this technique, is that Publisher's database is actually shared with other apps, so there is data within it that does not belong to Publisher—don't be surprised when querying if you see collections that you don't recognise.
+
+From within the Rails console, query the data using the Mongo shell:
+
+```ruby
+client = Mongo::Client.new(ENV["MONGODB_URI"])
+db = client.database
+
+db.collection_names
+=>
+  ["artefacts",
+    ...
+  ]
+```