How to extract concrete struct from interface

Question

I have multiple database tables with a concrete struct for each table. I'm using sqlx. My functions for database access work fine for each concrete struct.

Since many of my database operations (eg, insert a row) are identical across the tables I'd like to DRY them up a bit by having a common set of functions that take an interface and perform the operations.

In a function that takes the interface and calls sqlx functions (eg, sqlx.Get) I need to pull the concrete struct out of the interface to hand to sqlx. And I'm stumped there.

I've read the Laws of Reflection . I've read the reflect package documentation . I've looked all over and can't seem to put the pieces together properly.

Specific code:

type Persistable interface {
    DbFieldNames() []string
    DbPrepCreate() error
    TableName() string
}

type Session struct {} implements those methods and is accepted as a Persistable.

This statement works fine (with the concrete struct explicitly created.

// src is the Persistable passed into this function
dest := Session{}
err = stmt.Get(&dest, src)

But using that explicitly created struct in the common function defeats the purpose. What I want to do is this:

dest := SomeFunctionThatExtractsTheConcreteStruct( src ) # such that I can pass that dest into sqlx.

I've used the reflect package to examine the src variable (type, value, kind, ......). I can easily see that src contains a *Session. I can get the real value out. But creating a concrete class from src to pass into sqlx eludes me.

Answer 1

You have the variable named src with type Persistable containing a *Session . Your goal is to get an interface{} value containing a *Session to use as the first argument to the Get method.

Because Persistable is assignable to interface{} (as are all types), you can simply pass src to the first argument of the Get method.

err = stmt.Get(src, args...)