pulley/internal/data/movies.go

package data

import (
	"context"
	"errors"
	"time"

	"github.com/jackc/pgx/v5"
	"github.com/jackc/pgx/v5/pgconn"
	"github.com/jackc/pgx/v5/pgxpool"
)

// MUST export fields to serialize them

type Movie struct {
	ID        int64     `json:"id"`
	CreatedAt time.Time `json:"-"` // omits always
	Title     string    `json:"title"`
	Year      int32     `json:"year,omitzero"`
	// Runtime   int32     `json:"runtime,omitzero"`
	// Use the Runtime type instead of int32. Note that the omitzero directive will
	// still work on this: if the Runtime field has the underlying value 0, then it will
	// be considered zero and omitted -- and the MarshalJSON() method we just made
	// won't be called at all.
	// VSCode will complain though about reflection issue
	Runtime Runtime  `json:"runtime,omitzero`
	Genres  []string `json:"genres,omitzero"`
	Version int32    `json:"version"`
}

type MovieModel struct {
	pool *pgxpool.Pool
}

func (m MovieModel) Insert(ctx context.Context, movie *Movie) error {
	query := `
        INSERT INTO movies (title, year, runtime, genres) 
        VALUES ($1, $2, $3, $4)
        RETURNING id, created_at, version`

	args := []any{movie.Title, movie.Year, movie.Runtime, movie.Genres}

	row := m.pool.QueryRow(ctx, query, args...)
	// Insert is mutating the Movie struct
	err := row.Scan(&movie.ID, &movie.CreatedAt, &movie.Version)
	return err
}

func (m MovieModel) Get(ctx context.Context, id int64) (*Movie, error) {
	// safety validation
	if id < 1 {
		return nil, ErrRecordNotFound
	}

	query := `
		SELECT id, created_at, title, year, runtime, genres, version
		FROM movies
		WHERE id = $1
	`
	var movie Movie

	err := m.pool.QueryRow(ctx, query, id).Scan(
		&movie.ID,
		&movie.CreatedAt,
		&movie.Title,
		&movie.Year,
		&movie.Runtime,
		&movie.Genres,
		&movie.Version,
	)
	if err != nil {
		switch {
		case errors.Is(err, pgx.ErrNoRows):
			return nil, ErrRecordNotFound
		default:
			return nil, err
		}
	}

	return &movie, nil
}

func (m MovieModel) Update(ctx context.Context, movie *Movie) error {
	// Using version here as an optimistic lock. Look up optimistic vs pessimistic locking
	// https://stackoverflow.com/questions/129329/optimistic-vs-pessimistic-locking/129397#129397

	// version incrementing with an integer is cheap. Don't try with a last_updated timestamp...that could
	// cause issues. If you don't want version to be guessable then a UUID generated by the DB is suitable.
	// Example: SET ... version = uuid_generate_v4()

	query := `
		UPDATE movies 
		SET title = $1, year = $2, runtime = $3, genres = $4, version = version + 1
		WHERE id = $5 and version = $6
		RETURNING version	
	`

	args := []any{
		movie.Title,
		movie.Year,
		movie.Runtime,
		movie.Genres,
		movie.ID,
		movie.Version,
	}

	// Will not return any rows if the version number has already changed.
	err := m.pool.QueryRow(ctx, query, args...).Scan(&movie.Version)
	if err != nil {
		switch {
		case errors.Is(err, pgx.ErrNoRows):
			return ErrEditConflict
		default:
			return err
		}
	}

	return nil
}

// Here I decided to attempt an idiomatic version of the Delete service with
// a database transaction. Since this is updating just one entry in a single table
// an explicit db transaction isn't really necessary but I thought I'd practice it
// none the less. If using sqlc you'd probably not even worry about implementing
// it this way, but I am here to learn so.

func (m MovieModel) Delete(ctx context.Context, id int64) (err error) {

	if id < 1 {
		return ErrRecordNotFound
	}

	query := `DELETE FROM movies WHERE id = $1`

	tx, err := m.pool.BeginTx(ctx, pgx.TxOptions{})
	if err != nil {
		return err
	}
	defer func() {
		// If it's in error or the context was canceled, rollback
		if err != nil || ctx.Err() != nil {
			_ = tx.Rollback(ctx)
			return
		}
		// if the commit fails raises the error.
		err = tx.Commit(ctx)
	}()

	var cmd pgconn.CommandTag
	cmd, err = tx.Exec(ctx, query, id)
	if err != nil {
		return err
	}

	if cmd.RowsAffected() == 0 {
		return ErrRecordNotFound
	}

	return nil
}