How To Retrieve Relation Counts

General
1

In the console, child relations columns show a message similar to: “There are nnnn objects referencing this object as a relation.” Given that nnnn is known, is there a way to retrieve it when querying for records, hopefully in one call? I see that Count only works on “real” columns, not incoming relations.

The pattern I’m trying to implement is a common one, simply trying prevent deletion on the client of records that are pointed to by other records.

Thank you!

2

Hi @Kelly_Oglesby

Assume you have ParentTable with a relation column children to the table ChildTable.
Now if you want to know, how many parent objects are referencing specific child object as a relation, you should make the following query:

DataQueryBuilder queryBuilder = new DataQueryBuilder();
queryBuilder.whereClause =
    "children.objectId = 'YOUR_CHILD_OBJECT_ID'";
queryBuilder.properties = ["Count(objectId)"];
Backendless.data
    .of("ParentTable")
    .find(queryBuilder)
    .then((value) => print(value));

That is the Dart code but I hope you get the point. Let me know if you need the example in another language.

Also I would recommend to read this docs:
Search with the Where Clause
Search with SubQuery

Best Regards,
Maksym

3

Thank you, that is good to know, but I am trying to count the relations in the opposite direction. I want to download all of the child records, with counts of how many records in 1 to N parent tables have relations to the child records. If the count is > 0, the child record can’t be deleted. In the console, the counts are displayed in the various “RELATION FROM” columns.

Link to Foto

I know I can maintain my own counts (I can use your code snippet to get initial counts) or do queries on the parent tables, I was just thinking there might be a way to access those counts, as shown in this screen capture.

Thanks for your help!

4

Sorry, just one more example. Given the child table in this foto:

Link to Screenshot

what I would like to do, in pseudo code is:

queryBuilder.properties = [“size”, “Count(vehicles) as refs”]
Backendless.data.of(“ChildTable”).find(queryBuilder)

That throws an error that “vehicles” isn’t a valid column in ChildTable, which is true.

5

Yes, that is what my code does :slight_smile:

I guest your table is named Vehicle so the code should be:

queryBuilder.whereClause = “backupVehicle.objectId = ‘YOUR_CHILD_OBJECT_ID’”;
queryBuilder.properties = [“Count(objectId)”];
Backendless.data.of(“routes”).find(queryBuilder);

That will return you a count specified in “There are n objects referencing this object as a relation”.

In other words, you should make request to the routes table and look if any “parent” routes objects are referencing to your “child” Vehicle object.

6

If you have several Parent tables (routes and vehicles I think) you should make several requests to discover if any parent object is referencing to your child object.

7

Thank you, Maksym, that’s what I will do.

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