Error exporting data: Didn't understand relationship 'customObject__r' in FROM part of query call. If you are attempting to use a custom relationship

Question

I'm trying to export some data from my sandbox org for import into a scratch org, but I seem to be encountering an issue with my query:

MediaQualitySetJunction__c looks like this:

And this is the query I'm using:

SELECT
    Id,
    Name,
    (SELECT Id, Name, Width__c, Height__c, Resolution__c FROM MediaQuality__r)
FROM MediaQualitySetJunction__c

The full error:

Didn't understand relationship 'mediaquality__r' in FROM part of query call. If you are attempting to use a custom relationship, be sure to append the '__r' after the custom relationship name. Please reference your WSDL or the describe call for the appropriate names.

I'm referencing MediaQuality by the its name on the junction object (with the __r suffix). I've also tried it without this suffix, and with the "Child Relationship Name" I see in Salesforce (MediaQualityJunction), with and without the __r, and I still get the same error.

Based on sfdcfox's answer, I've tried the following export:

sfdx force:data:tree:export -u "[email protected]" -q "SELECT Id, Name, MediaQuality__r.Id, MediaQuality__r.Name, MediaQuality__r.Width__c, MediaQuality__r.Height__c, MediaQuality__r.Resolution__c, MediaQualitySet__r.Id, MediaQualitySet__r.Name FROM MediaQualitySetJunction__c"

Which produces the following JSON:

{
    "records": [
        {
            "attributes":
            {
                "type": "MediaQualitySetJunction__c",
                "referenceId": "MediaQualitySetJunction__cRef1"
            },
            "Name": "000035",
            "MediaQuality__r":
            {
                "attributes":
                {
                    "type": "MediaQuality__c",
                    "url": "/services/data/v50.0/sobjects/MediaQuality__c/a020p000001WR5oAAG"
                },
                "Id": "a020p000001WR5oAAG",
                "Name": "1080p",
                "Width__c": 1920,
                "Height__c": 1080,
                "Resolution__c": "1920x1080"
            },
            "MediaQualitySet__r":
            {
                "attributes":
                {
                    "type": "MediaQualitySet__c",
                    "url": "/services/data/v50.0/sobjects/MediaQualitySet__c/a010p000001O3YZAA0"
                },
                "Id": "a010p000001O3YZAA0",
                "Name": "540p, 720p, 1080p (sandbox)"
            }
        }
    ]
}

And then executed the following to import it into my scratch org:

sfdx force:data:tree:import -u [email protected] -f "MediaQualitySetJunction__c.json"

I've confirmed that I have access to all of the fields involved and that the field visibility is checked for the System Administrator. I get the following result from the above command:

{
    "hasErrors": true,
    "results": [
        {
            "referenceId": "MediaQualitySetJunction__cRef1",
            "errors": [
                {
                    "statusCode": "INVALID_FIELD",
                    "message": "Cannot reference a foreign key field MediaQuality__r.",
                    "fields": []
                },
                {
                    "statusCode": "INVALID_FIELD",
                    "message": "Cannot reference a foreign key field MediaQualitySet__r.",
                    "fields": []
                }
            ]
        }
    ]
}

How do I solve this?

Answer 1

MediaQuality__c is the apparently parent of MediaQualitySetJunction__c, so you should be querying it as a parent relationship:

SELECT
    Id,
    Name,
    MediaQuality__r.Name, 
    MediaQuality__r.Width__c, 
    MediaQuality__r.Height__c, 
    MediaQuality__r.Resolution__c 
FROM MediaQualitySetJunction__c