Elasticsearch for Django

This is a lightweight Django app for people who are using Elasticsearch with Django, and want to manage their indexes.

Search Index Lifecycle

The basic lifecycle for a search index is simple:

Create an index
Post documents to the index
Query the index

Relating this to our use of search within a Django project it looks like this:

Create mapping file for a named index
Add index configuration to Django settings
Map models to document types in the index
Post document representation of objects to the index
Update the index when an object is updated
Remove the document when an object is deleted
Query the index
Convert search results into a QuerySet (preserving relevance)

Django Implementation

This section shows how to set up Django to recognise ES indexes, and the models that should appear in an index. From this setup you should be able to run the management commands that will create and populate each index, and keep the indexes in sync with the database.

Create index mapping file

The prerequisite to configuring Django to work with an index is having the mapping for the index available. This is a bit chicken-and-egg, but the underlying assumption is the you are capable of creating the index mappings outside of Django itself, as raw JSON - e.g. using the Chrome extension Sense, or the API tool Paw. (The easiest way to spoof this is to POST a JSON document representing your document type at URL on your ES instance (POST http://ELASTICSEARCH_URL/{{index_name}}) and then retrieving the auto-magic mapping that ES created via GET http://ELASTICSEARCH_URL/{{index_name}}/_mapping.)

Once you have the JSON mapping, you should save it as search/mappings/{{index_name}}.json.

Configure Django settings

The Django settings for search are contained in a dictionary called SEARCH_SETTINGS, which should be in the main django.conf.settings file. The dictionary has three root nodes, connections, indexes and settings. Below is an example:

SEARCH_SETTINGS = {
    'connections': {
        'default': getenv('ELASTICSEARCH_URL'),
    },
    'indexes': {
        'blog': {
            'models': [
                'website.BlogPost',
            ]
        }
    },
    'settings': {
        # batch size for ES bulk api operations
        'chunk_size': 500,
        # default page size for search results
        'page_size': 25,
        # set to True to connect post_save/delete signals
        'auto_sync': True,
        # if true, then indexes must have mapping files
        'strict_validation': False
    }
}

The connections node is (hopefully) self-explanatory - we support multiple connections, but in practice you should only need the one - 'default' connection. This is the URL used to connect to your ES instance. The setting node contains site-wide search settings. The indexes nodes is where we configure how Django and ES play together, and is where most of the work happens.

Index settings

Inside the index node we have a collection of named indexes - in this case just the single index called blog. Inside each index we have a models key which contains a list of Django models that should appear in the index, denoted in app.ModelName format. You can have multiple models in an index, and a model can appear in multiple indexes. How models and indexes interact is described in the next section.

Configuration Validation

When the app boots up it validates the settings, which involves the following:

Do each of the indexes specified have a mapping file?
Do each of the models implement the required mixins

Implement search document mixins

So far we have configure Django to know the names of the indexes we want, and the models that we want to index. What it doesn't yet know is which objects to index, and how to convert an object to its search index document. This is done by implementing two separate mixins - SearchDocumentMixin and SearchDocumentManagerMixin. The configuration validation routine will tell you if these are not implemented.

SearchDocumentMixin

This mixin must be implemented by the model itself, and it requires a single method implementation - as_search_document(). This should return a dict that is the index representation of the object; the index kwarg can be used to provide different representations for different indexes. By default this is _all which means that all indexes receive the same document for a given object.

def as_search_document(self, index='_all'):
    return {name: “foo”} if index == 'foo' else {name = “bar”}

SearchDocumentManagerMixin

This mixin must be implemented by the model's default manager (objects). It also requires a single method implementation - get_search_queryset() - which returns a queryset of objects that are to be indexed. This can also use the index kwarg to provide different sets of objects to different indexes.

def get_search_queryset(self, index):
    return self.get_queryset().filter(foo="bar")

We now have the bare bones of our search implementation. We can now use the included management commands to create and populate our search index:

# create the index 'foo' from the 'foo.json' mapping file
$ ./manage.py create_search_index foo

# populate foo with all the relevant objects
$ ./manage.py update_search_index foo

The next step is to ensure that our models stay in sync with the index.

Add model signal handlers to update index

If the setting auto_sync is True, then on AppConfig.ready each model configured for use in an index has its post_save and post_delete signals connected. This means that they will be kept in sync across all indexes that they appear in whenever the relevant model method is called. (There is some very basic caching to prevent too many updates - the object document is cached for one minute, and if there is no change in the document the index update is ignored.)

There is a VERY IMPORTANT caveat to the signal handling. It will only pick on changes the the model itself, and not on related (ForeignKey, ManyToManyField) model changes. If the search document it affected by such a change then you will need to implement additional signal handling yourself.

We now have documents in our search index, kept up to date with their Django counterparts. We are ready to start querying ES.

Search Queries (How to Search)

Running search queries

The search itself is done using elasticsearch_dsl, which provides a pythonic abstraction over the QueryDSL, but also allows you to use raw JSON if required:

from elasticsearch_django.settings import get_client
from elasticsearch_dsl import Search

# run a default match_all query
search = Search(using=get_client())
response = search.execute()

# change the query using the python interface
search = search.query("match", title="python")

# change the query from the raw JSON
search.update_from_dict({"query": {"match": {"title": "python"}}})

The response from execute is a Response object which wraps up the ES JSON response, but is still basically JSON.

SearchQuery

The elasticsearch_django.models.SearchQuery model wraps this functionality up and provides helper properties, as well as logging the query:

from elasticsearch_django.settings import get_client
from elasticsearch_django.models import SearchQuery
from elasticsearch_dsl import Search

# run a default match_all query
search = Search(using=get_client(), index='blog')
sq = SearchQuery.execute(search)

Calling the SearchQuery.execute class method will execute the underlying search, log the query JSON, the number of hits, and the list of hit meta information for future analysis. The execute method also includes these additional kwargs:

user - the user who is making the query, useful for logging
reference - a free text reference field - used for grouping searches together - could be session id, or brief id.
save - by default the SearchQuery created will be saved, but passing in False will prevent this.

In conclusion - running a search against an index means getting to grips with the elasticsearch_dsl library, and when playing with search in the shell there is no need to use anything else. However, in production, searches should always be executed using the SearchQuery.execute method.

Converting search hits into Django objects

Running a search against an index will return a page of results, each containing the _source attribute which is the search document itself (as created by the SearchDocumentMixin.as_search_document method), together with meta info about the result - most significantly the relevance score, which is the magic value used for ranking (ordering) results. However, the search document probably doesn't contain all the of the information that you need to display the result, so what you really need is a standard Django QuerySet, containing the objects in the search results, but maintaining the order. This means injecting the ES score into the queryset, and then using it for ordering. There is a method on the SearchDocumentManagerMixin called from_search_query which will do this for you. It uses raw SQL to add the score as an annotation to each object in the queryset. (It also adds the 'rank' - so that even if the score is identical for all hits, the ordering is preserved.)

from models import BlogPost

# run a default match_all query
search = Search(using=get_client(), index='blog')
sq = SearchQuery.execute(search)
for obj in BlogPost.objects.from_search_query(sq):
    print obj.search_score, obj.search_rank

ipsolar/elasticsearch-django