How To Store MongoDB Documents In A Slice And Iterate Over Them In Golang

Introduction to creating MongoDB queries and filters in Golang

If you’re working with MongoDB and Golang, it’s important to know how to create and format MongoDB filter queries so that they can be passed into method calls. Fortunately, this process is fairly simple and straightforward. In this article, we’ll show you how to create BSON map objects from raw JSON strings and from primitive.M BSON objects. We’ll also explain how to pass these objects to the Golang driver’s method calls to query MongoDB documents.

Prerequisites for making API calls to MongoDB using the official Golang driver

Before we dive into the Golang code and learn how to make API calls to MongoDB, there are a few prerequisites that need to be reviewed:

• MongoDB needs to be installed and running on the same server that’s making the Go API calls. You can type mongod in your terminal window to get information on the MongoDB daemon processes.

• You’ll need to have a few documents in a MongoDB collection that you can query in your Go script:

• If you’d like to access the attributes and fields of a Golang object, you’ll need to use a third-party library called oleiade/reflections, which returns high-level abstractions of Go’s native reflect library. To install this package, enter the following commands in your terminal or command prompt window:

• Golang also needs to be installed, and the MongoDB project directory needs to be in Go’s $GOPATH:

You can get information on the current value of GOPATH by using the commands go help gopath or $GOPATH. For more help with Go’s get command, enter go help get into a terminal window.

Import the MongoDB and Go packages in a Golang script

Now that we’ve gone over the prerequisites, we’re ready to turn to the code. We’ll start our script by importing the packages for MongoDB that can create BSON filters and queries and unmarshal strings in the main() function:

Declare a new struct for the MongoDB document fields

We’ll use Golang’s struct datatype to map the MongoDB fields for the collection’s documents:

View a collection’s document fields in the Mongo Shell

If you’re not sure of the datatypes for a collection’s document fields, simply enter these commands in the Mongo Shell to view the exact format of the fields:

Declare the Golang main() function and connect to MongoDB

Now we’ll declare the main() function, which is necessary for the MongoDB API calls and document iteration. We can connect to the MongoDB server using mongo.Connect() and have it return a client instance that will be used to get a collection’s documents:

When you’re adding this code to your script, be sure to replace the "SomeDatabase" and "Some Collection" strings shown in the example with the correct names for your MongoDB database and collection.

Declaring a BSON map MongoDB filter object

Next, we’ll nest a BSON query into another BSON map object which has one of the collection’s fields as its key:

Build a MongoDB filter query from a string in Golang

In this section, we’ll look at the process of creating a BSON map object from a string to use for a MongoDB query in Golang.

Declare a Go string containing a MongoDB JSON filter

We’ll use backticks to declare a raw string of a valid JSON object in Golang; this MongoDB JSON string in Golang will be used for the filter. Then, we’ll instantiate an empty bson.M object for the unmarshaled data to be stored in:

Unmarshal a Go string using the JSON package

After that, we’ll unmarshal our string and return a primitive.M object:

Declare a nested bson.M MongoDB filter from the primitive.M object returned by json.Unmarshal()

Use the MongoDB filter to return a Cursor object and iterate over the documents

At this point, we’ll have the Golang driver’s Find() method return a Cursor object. Then we’ll iterate over the documents by passing a context object to the cursor’s Next() method:

Conclusion

Being able to construct filter queries is essential if you want to harness the power of MongoDB in your Golang scripts. Using strings and BSON map objects, it’s easy to work with MongoDB filters in Golang. With the help of the examples provided in this tutorial, you’ll have a better understanding of how to create a MongoDB filter query using BSON in Golang.

Just the Code

We’ve looked at our code one section at a time so far. Shown below is the entire script used to query for MongoDB documents with BSON in Golang: