Eventually, it seems, looking up data in multiple MongoDB collections at the same time becomes necessary. Okay, necessary, might be strongly phrased. “Very helpful” may be better. Until version 3.2 of MongoDB doing joins wasn’t possible, but with that version came the $lookup
aggregation stage operator. The introduction of $lookup
allows for left outer joins to be performed on collections in the same database which are not sharded.
For our data for this post let’s utilize the concept of recipes. I’ll lay out some basic traditional SQL database tables and walk through the SQL syntax for a join.
SQL Syntax
Let’s have a quick look at what a left outer join looks like in SQL with a couple of different syntaxes in SQL.
Standard SQL
SELECT recipe_type.decription, recipes.title FROM recipe_type LEFT OUTER JOIN recipes ON recipe_type.id = recipes.recipe_type_id
Oracle
SELECT recipe_type.description, recipes.title FROM recipe_type, recipes WHERE recipe_type.id = recipes.recipe_type_id(+)
The results of a left outer join for these two tables will contain all of the rows from our “left” table (recipe_type).
$lookup for Joins
Our join from $lookup
then should be the same from a conceptual standpoint as our SQL cousin. Assuming we have a collection of recipe types and one of recipes our $lookup
stage would look like the following:
{ $lookup: { from: "recipes", localField: "id", foreignField: "recipe_type_id", as: "recipe_categories" } }
Our $lookup
stage then is doing an equality match between the two documents based on the id
and recipe_type_id
fields. It will add a new array field to the document in our aggregation pipeline document and passes it to the next stage of the pipeline.
Now, given our knowledge of schema design and document models in MongoDB we may not have a need for this exact join as these two collections of data might be embedded in one or the other collection. However, depending on your data access patterns and application needs, the collections may be utilizing references.
Wrap Up
I hope this provided a useful, and quick, introduction to the $lookup
aggregation stage and how it is possible to do joins in MongoDB. While you may not find yourself using it all of the time, it is a great tool to have at the ready when it is needed.
There are several MongoDB specific terms in this post. I created a MongoDB Dictionary skill for the Amazon Echo line of products. Check it out and you can say “Alexa, ask MongoDB what is a document?” and get a helpful response.
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