Why Do I Get an 'Undefined Method' Error When Calling Methods on an Interface Pointer in Go?

Calling Methods on Interface Pointers in Golang

When working with the Gorp library for database access, developers often encounter the need to call methods on an interface pointer while maintaining the ability to roll back transactions. This issue arises from the requirement to pass the Gorp DbMap or Transaction instance as a field property.

The Problem

As demonstrated in the example code, attempting to call methods on the SqlExecutor interface using a pointer reference results in an undefined method error. This occurs because the type *gorp.SqlExecutor does not have a Get method, while the gorp.SqlExecutor interface itself does.

The Solution

The underlying misconception lies in the assumption that passing a pointer to an interface value provides the same behavior as call by reference in other languages. In Go, pointers to interfaces are used for two specific reasons:

1. To optimize performance when passing large struct values.
2. To allow the callee to modify the original value (when the method has a pointer receiver).

Neither of these reasons applies to pointers to interfaces, as interface values are already tiny and cannot be modified through the interface value itself. Instead, it is necessary to modify the value stored inside the interface value, which is often a pointer to an actual struct.

Correct Implementation

To resolve the issue, the code should avoid passing a pointer to the Gorp "object" and instead pass the interface value directly. The following code correctly implements this approach:

package repositories

import "github.com/coopernurse/gorp"

type Repository struct {
    Gorp gorp.SqlExecutor // Pass the interface value directly
}

func (r Repository) GetById(i interface{}, key interface{}) interface{} {
    obj, err := r.Gorp.Get(i, key)
    if err != nil {
        panic(err)
    }
    return obj
}

By passing the interface value by value and not by reference, the code can maintain the required functionality while adhering to the proper usage of pointers in Go.

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