Nested structs of arrays in Go

Question

I have started using https://mholt.github.io/json-to-go/ to turn API JSON into go structs and I really like it, however I am stuck on how to initialize the Filters array struct in the Report Definition struct shown below.

type ReportDefinition struct {
    ReportName string `json:"reportName"`
    ReportType string `json:"reportType"`
    Product    string `json:"product"`
    Email      string `json:"email"`
    CreatedBy  string `json:"createdBy"`
    Duration   struct {
        Duration  string      `json:"duration"`
        StartDate interface{} `json:"startDate"`
        EndDate   interface{} `json:"endDate"`
    } `json:"duration"`
    Filters []struct {
        Column struct {
            ColumnName string `json:"columnName"`
            Value      string `json:"value"`
        } `json:"column"`
        Operator string `json:"operator"`
    } `json:"filters"`
    SortBy            interface{}   `json:"sortBy"`
    ReportGroup       interface{}   `json:"reportGroup"`
    ReportOnHierarchy bool          `json:"reportOnHierarchy"`
    IsReportPreview   string        `json:"isReportPreview"`
    OutputRecordCount interface{}   `json:"outputRecordCount"`
    Schedule          interface{}   `json:"schedule"`
    Columns           []interface{} `json:"columns"`
}

I cannot seem to reference the items declared within the Filters struct or even the Filters struct in order to create a new Filter item and append it to Filters.

Is it possible to reference the Column struct with the ReportDefinition written as is? Or am I am doing something silly and I should just declare Filters outside of the ReportDefinition structure?

Answer 1

Since the sliced type in the Filters field doesn't have a name, you have to repeat the definition when you want to initialize it:

package main

import "fmt"

type ReportDefinition struct {
    Filters []struct {
            Column struct {
                    ColumnName string `json:"columnName"`
                    Value      string `json:"value"`
            } `json:"column"`
            Operator string `json:"operator"`
    } `json:"filters"`
}

func main() {
    var rd ReportDefinition
    rd.Filters = append(rd.Filters, struct {
            Column struct {
                    ColumnName string `json:"columnName"`
                    Value      string `json:"value"`
            } `json:"column"`
            Operator string `json:"operator"`
    }{
            Column: struct {
                    ColumnName string `json:"columnName"`
                    Value      string `json:"value"`
            }{
                    ColumnName: "foo",
                    Value:      "bar",
            },
            Operator: "==",
    })

    fmt.Printf("+%v\n", rd)
}

It should be clear that this is very unconvenient, so you should just give your types names (at least those that you want to initialize):

package main

import "fmt"

type ReportDefinition struct {
    Filters []Filter `json:"filters"`
}

type Filter struct {
    Column   Column `json:"column"`
    Operator string `json:"operator"`
}

func NewFilter(col, op, val string) Filter {
    return Filter{
            Column: Column{
                    ColumnName: col,
                    Value:      val,
            },
            Operator: op,
    }
}

type Column struct {
    ColumnName string `json:"columnName"`
    Value      string `json:"value"`
}

func main() {
    var rd ReportDefinition
    rd.Filters = append(rd.Filters, NewFilter("foo", "==", "bar"))

    fmt.Printf("+%v\n", rd)
}