Panic: interface conversion: * is not inteface X: missing method xxx

I'm working on a project that consume messages from Kafka, unmushal using messagepack or json format, build sql of it and insert them into TDengine database.

The inteface defined as:

type TaosEncoder interface {
	TaosDatabase() string
	TaosSTable() string
	TaosTable() string
	TaosTags() []interface{}
	TaosCols() []string
	TaosValues() []interface{}
}

For an object called Record:

type Record struct {
	DeviceID  string        `json:"deviceID"` // for table name
	Timestamp time.Time     `json:"timestamp"`
	Cols      []interface{} `json:"cols"`
}

Implement the interface:

func (r *Record) TaosCols() []string {
	var tags []string
	return tags
}
// other methods are implemented too

A method will use the interface methods:

func ToTaosBatchInsertSql(l []interface{}) (string, error) {
	records := make([]string, len(l))
	for i, t := range l {
		// type TaosEncoderT = *TaosEncoder
		record, err := ToTaosRecordSql(t.(TaosEncoder))
		if err != nil {
			return "", err
		}
		records[i] = record
	}
	return fmt.Sprintf("insert into %s", strings.Join(records, " ")), nil
}

Use it as this:

records := make([]interface{}, size)
for i := 0; i < size; i++ {
	item := <-q.queue # an channel of Record, defined as: queue chan interface{}
	records[i] = item
}
sqlStr, err := utils.ToTaosBatchInsertSql(records)

It compiles ok, but failed while running like this:

panic: interface conversion: record.Record is not utils.TaosEncoder: missing method TaosCols
goroutine 71 [running]:
xxx/pkg/utils.ToTaosBatchInsertSql(0xc000130c80, 0xc8, 0xc8, 0xc8, 0x0, 0x0, 0x0)

But when I change the implementations of Record - remove *

func (r Record) TaosCols() []string {
	var tags []string
	return tags
}

Then it just works.

I've told to use * in interface implements, so the question is:

1. Is it ok to implement interface with func (r Record) xxx?
2. If not, what can I do?

The cause of this error is related to the method sets of a type.

Assume a type T that has methods with a receiver type of T and some more methods with a receiver type of *T.

Then, if a variable is of type T, it has all methods available that have a pointer receiver of T but none of the methods with receiver type *T.

If a variable is of type *T, it has both sets of methods available, those with receiver type T, and those with receiver type *T.

In your case, type Record has no TaosCols() method, and so it does not implement interface TaosEncoder completely.

If you turn the variable of type Record into a *Record variable, it has all methods available and then would implement the interface.

Method Sets in: Frequently Asked Questions (FAQ) - The Go Programming Language

Method Sets in: The Go Programming Language Specification - The Go Programming Language

A Record is not the same type as a *Record. If you use a pointer receiver for defining the methods that implement the interface, then only *Record implements TaosEncoder.

Based on this:

item := <-q.queue # an channel of Record

it looks like you will need to send a value of *Record on q.queue, rather than a Record.