Unexpected Behavior When Merging Structs Without Override

[sourabh-khot65]

When using mergo.Merge with override set to false, non-empty fields in the destination struct are being overwritten by values from the source struct. This behavior contradicts the expected functionality where only empty fields in the destination should be updated.

package main

import (
	"fmt"
	"github.com/imdario/mergo"
)

type Candidate struct {
	FirstName *string
	LastName  *string
	Skills    *string
}

func ptr(s string) *string {
	return &s
}

func main() {
	dest := &Candidate{
		FirstName: ptr("John"),
		LastName:  ptr("Doe"),
		Skills:    nil, // Empty destination field
	}

	src := &Candidate{
		LastName: ptr("Smith"),
		Skills:   ptr("Go"), // Source has a value
	}

	// Merge without override
	err := mergo.Merge(dest, src)
	if err != nil {
		fmt.Println("Error:", err)
		return
	}

	fmt.Printf("Merged Struct: %+v\n", dest)
}

Expected Behavior:

&Candidate{
    FirstName: "John",
    LastName:  "Doe", // Should not be overwritten
    Skills:    "Go",  // Should be filled
}

Actual Behavior:

&Candidate{
    FirstName: "John",
    LastName:  "Smith", // Overwritten unexpectedly
    Skills:    "Go",
}

This behavior is unexpected, as mergo.Merge should only populate empty fields in dest when WithOverride is not used. Could you clarify if this is intended behavior or provide a solution? Thank you!

[darccio]

Sorry, I just saw this. I'll check it this week, @sourabh-khot65.

[darccio]

@sourabh-khot65 I'm afraid your code doesn't reproduce the issue: https://go.dev/play/p/U5rnLXo5YlI

package main

import (
	"fmt"

	"dario.cat/mergo"
)

type Candidate struct {
	FirstName *string
	LastName  *string
	Skills    *string
}

func ptr(s string) *string {
	return &s
}

func main() {
	dest := &Candidate{
		FirstName: ptr("John"),
		LastName:  ptr("Doe"),
		Skills:    nil, // Empty destination field
	}

	src := &Candidate{
		LastName: ptr("Smith"),
		Skills:   ptr("Go"), // Source has a value
	}

	// Merge without override
	err := mergo.Merge(dest, src)
	if err != nil {
		fmt.Println("Error:", err)
		return
	}

	fmt.Printf("FirstName: %q; LastName: %q; Skills: %q\n", *dest.FirstName, *dest.LastName, *dest.Skills)
}

I only modified the fmt.Printf call, which outputs:

FirstName: "John"; LastName: "Doe"; Skills: "Go"

[sourabh-khot65]

@darccio Thank you for reaching out. It seems there was an implementation issue on my end, which was later resolved.