include relation in mongo query

To query for related documents in MongoDB using the Go language, you can make use of the $lookup operator with the pipeline option. Here is an example query:

pipeline := []bson.M{
    bson.M{
        "$lookup": bson.M{
            "from":         "<related_collection>",
            "localField":   "<local_field>",
            "foreignField": "<foreign_field>",
            "as":           "<output_field>",
        },
    },
}
cursor, err := collection.Aggregate(context.TODO(), pipeline)

Let's go through each step of the query:

1. Define the pipeline: Create a variable called pipeline and initialize it as an array of bson.M (MongoDB's BSON document type). This array will contain the stages of the aggregation pipeline.

2. $lookup stage: Inside the pipeline array, add a bson.M document that represents the $lookup stage. This stage allows you to perform a left outer join on two collections.

3. Specify the related collection: In the $lookup stage, set the "from" field to the name of the related collection you want to query against.

4. Define the local field: Set the "localField" field to the name of the field in the current collection that you want to match with the foreign field in the related collection.

5. Define the foreign field: Set the "foreignField" field to the name of the field in the related collection that you want to match with the local field.

6. Specify the output field: Set the "as" field to the name of the field where the related documents will be stored in the output.

7. Execute the query: Use the Aggregate method on your collection object, passing in the context and the pipeline variable. This will return a cursor that you can iterate over to access the result documents.

Remember to replace <related_collection>, <local_field>, <foreign_field>, and <output_field> with the appropriate values for your specific use case.

I hope this helps! Let me know if you have any further questions.