Rails - Export multiple models to CSV - ruby

Question

I am building a tool used for producing research reports. The tool produces reports with multiple models and nested models to display tabular data, and a report is structured something like this
Report
- main report attributes from report model
Nested Model 1
- nested model 1 attributes
Nested Model 2
- nested model 2 attributes
Right now I am exporting these reports to PDF, but I have gotten feature requests from users for exporting reports to Excel and CSV.
In my experience, CSV export has typically been good for "flat" associations, such as exporting a list of name/attributes from 1 model.
But if you have a complex report with multiple models, how do you structure an Excel / CSV file with data from multiple models?
Any examples of how you've structured something like this would be greatly appreciated.

Answer 1

If the attributes of the nested model you want to list in the CSV file are all has_one relation, it is not complicated that you can just append the nested model attributes right to the main model, report.
attr1, attr2, attr3,...,attrN, nested_attr1, nested_attr2...
For has_many relation, it is the similar solution but a little bit more complicated. First, you need to find the maximum number of the nested models belongs to a report. For example, if a report has many nested_reports, you need to check each report and use .count to find the number of the nested_reports and then choose the biggest one. Once you found the max number X, you can make a CSV like this:
attr1,...,attrN, nested_report1_attr1...nested_report1_attrN, nested_report2_attr1...nested_report2_attrN,...nested_reportX_attr1...nested_reportX_attrN
If a report only has one nested_report, the nested_report2...nested_reportX can just be left blank.
If the report has multiple nested models, then just append them as the above.
If you have a large number of data, finding the max number of nested model of the report will be a bottleneck. You may need an elaborated JOIN SQL to optimize it. However, I personally choose another solution: use counter_cache, so I can find that number as fast as possible by using:
Report.all.maximum(:nested_model_count)
You can check https://guides.rubyonrails.org/association_basics.html#options-for-belongs-to-counter-cache about how to use :counter_cache