A simple CRUD library for PostgreSQL with generics in Go
Tags: programming, go
Published: 2022-10-25 11:12:46

I've [written](https://andrewpillar.com/programming/2019/07/13/orms-and-query-building-in-go) previously about my thoughts on ORMs in Go and how the typical Active Record style ORMs seem to be a poor fit as a whole. With the advent of generics however, I've decided to revisit this topic, and see how generics could be utilied to make working with databases easier when it comes to modelling data. The parts I'm mostly interested in is providing a thin abstraction over the typical CRUD (create, read, update, delete) operations an ORM would provide for each model. Ideally, we want to provide this abstraction in a way that makes as few assumptions about the data being modelled as possible, and that builds on top of what has already been explored with regards to query building. I'm not going to cover what generics are in this post, as the authors of the language have [already done that](https://go.dev/blog/intro-generics), instead we're going to dive right in.

>**Note:** For what we're going to be building we will be utilising the Go
libraries `github.com/jackc/pgx` and `github.com/andrewpillar/query`.

* [Models and stores](#models-and-stores)
* [Model creation](#model-creation)
* [Model updating](#model-updating)
* [Model deletion](#model-deletion)
* [Model reading](#model-reading)
* [Implementing a model](#implementing-a-model)
* [Conclusion](#conclusion)

## Models and stores

Pretty much every ORM has a concept of a model, that is a structure of data that represents some data from a database. These typically offer the ability to seamlessy create, update, and delete records of data that they map to. For our library, we will also want a way of representing our data too, and a mechanism by which to perform CRUD operations. For this, we will have the concept of a Model, for modelling data, and a Store, for performing the CRUD operations. So, let's implement both of these, first the Model,

    type ScanFunc func(dest ...any) error

    type Model interface {
        Primary() (string, any)

        Scan(fields []string, scan ScanFunc) error

        Params() map[string]any
    }

the Model is implemented as an interface with the methods `Primary`, `Scan`, and `Params`. The `Primary` method would return the name of the column used as the primary key for the Model, and the value for that column if any. The `Scan` method would be invoked when scanning data from the Model's underlying table into said Model, this would be given the fields being scanned, and the function to call to actually perform the scan. The `Params` method to return the parameters of the Model to be used during create and update operations. With the Model defined, we can use this as a type parameter for our Store implementation,

    type Store[M Model] struct {
        *pgxpool.Pool

        table string
        new   func() M
    }

    func NewStore[M Model](pool *pgxpool.Pool, table string, new func() M) *Store[M] {
        return &Store[M]{
            Pool:  pool,
            table: table,
            new:   new,
        }
    }

the Store implementation is a struct with a type parameter for anything that implements a Model. This will allow for the same Store to be used across multiple implementations of a Model interface. This takes a PostgreSQL connection pool from which to take connections from, a table, which is the table we would operate on, and a callback function for instantiating new Models, this would be used when scanning models from the database.

## Model creation

With the basic implementation of both Model and Store done, we can now expand the Store to support create operations for any Model we give it. First, let's implement the query building part of creation,

    func (s *Store[M]) Create(ctx context.Context, m M)) error {
        p := m.Params()

        cols := make([]string, 0, len(p))
        vals := make([]any, 0, len(p))

        for k, v := range p {
            cols = append(cols, k)
            vals = append(vals, v)
        }

        primary, _ := m.Primary()

        q := query.Insert(
            s.table,
            query.Columns(cols...),
            query.Values(vals...),
            query.Returning(primary),
        )
    ...

in this bit, we iterate over the parameters returned from `m.Params` and place them in `[]string` and `[]any` slices for the columns and values respectively. We also call `m.Primary` to get the name of the primary key column which we want returning from the query, this will allow us to scan the value back into the created model if it doesn't already have it, for example if the column is an auto-incrementing integer. Now, let's implement the rest where we invoke the query, and scan the results back in,

    ...
        rows, err := s.Query(ctx, q.Build(), q.Args()...)

        if err != nil {
            return err
        }

        defer rows.Close()

        if !rows.Next() {
            if err := rows.Err(); err != nil {
                return err
            }
            return nil
        }

        if err := m.Scan(s.fields(rows), rows.Scan); err != nil {
            return nil
        }
        return nil
    }

here, we pass the built query to the underlying database pool and scan the row that is returned. You'll notice how we invoke `s.fields` to get the fields from the rows that are returned, this is something that needs to be implemented so let's do that next,

    func (s *Store[M]) fields(rows pgx.Rows) []string {
        descs := rows.FieldDescriptions()
        fields := make([]string, 0, len(descs))

        for _, d := range descs {
            fields = append(fields, string(d.Name))
        }
        return fields
    }

the reason we pass the field names to the `Scan` method, is so that the Model will know which fields are being scanned. This reduces the assumptions that the Store has to make about the Model and its fields, and opens up the ability for more flexibility on the implementation of the Model.

## Model updating

Next, let's implement the logic for updating a Model. This will be somewhat
similar to what we did for creation,

    func (s *Store[M]) Update(ctx context.Context, m M) error {
        p := m.Params()

        opts := make([]query.Option, 0, len(p))

        for k, v := range p {
            opts = append(opts, query.Set(k, query.Arg(v)))
        }

        primary, id := m.Primary()

        opts = append(opts, query.Where(col, "=", query.Arg(id)))

        if _, err := s.Exec(ctx, q.Build(), q.Args()...); err != nil {
            return err
        }
        return nil
    }

simpler than what we did for create. Here, we use the returned values from the `Primary` method to construct the `WHERE` clause to ensure only that Model will be updated. We did one assumption though, we assumed that there would be no data that needs to be scanned back into the model, assuming that the Model given to us at this point in time contains all the necessary data for the update.

## Model deletion

Now, let's implement the logic for deleting a Model. Again, this will be similar
to what we did for update, but simpler,

    func (s *Store[M]) Delete(ctx context.Context, m M) error {
        col, id := m.Primary()

        q := query.Delete(s.table, query.Where(col, "=", query.Arg(id)))

        if _, err := s.Exec(ctx, q.Build(), q.Args()...); err != nil {
            return err
        }
        return nil
    }

again, we use `Primary` to construct the `WHERE` clause for that specific Model.

## Model reading

With the create, update, and delete operations implemented, we now need to implement the read operations. First, let's implement the `Select` and `All` methods for reading multiple Models,

    func (s *Store[M]) Select(ctx context.Context, cols []string, opts ...query.Option) ([]M, error) {
        opts = append([]query.Option{
            query.From(s.table),
        }, opts...)

        q := query.Select(query.Columns(cols...), opts...)

        rows, err := s.Query(ctx, q.Build(), q.Args()...)

        if err != nil {
            return nil, err
        }

        defer rows.Close()

        fields := s.rows(fields)

        mm := make([]M, 0)

        for rows.Next() {
            m := s.new()

            if err := m.Scan(fields, rows.Scan); err != nil {
                return nil, err
            }
        }

        if err := rows.Err(); err != nil {
            return nil, err
        }
        return mm, nil
    }

    func (s *Store[M]) All(ctx context.Context, opts ...query.Option) ([]M, error) {
        return s.Select(ctx, []string{"*"}, opts...)
    }

the `Select` method is what does most of the leg work, and will only select the given columns via `cols`, useful if you only want to load in a handful of parameters for a given Model. `Select` also makes use of the `s.new` callback for creating new Models to be scanned. The `All` method simply calls `Select` and specifies that every column should be selected.

With both `Select` and `All` implemented for returning multiple Models, let's now implement `Get` for returning a single Model,

    func (s *Store[M]) Get(ctx context.Context, opts ...query.Option) (M, bool, error) {
        var zero M

        opts = append([]query.Option{
            query.From(s.table),
        }, opts...)

        q := query.Select(query.Columns(cols...), opts...)

        rows, err := s.Query(ctx, q.Build(), q.Args()...)

        if err != nil {
            return zero, false, err
        }

        defer rows.Close()

        if !rows.Next() {
            if err := rows.Err(); err != nil {
                return zero, false, err
            }
            return zero, false, nil
        }

        m := s.new()

        if err := m.Scan(s.fields(rows), rows.Scan); err != nil {
            return zero, false, err
        }
        return m, true, nil
    }

for `Get` we return a `bool` as the second value, this is used as a simple flag to denote if there was any Model that was found. At the very top of the method we define `var zero M`, this is simply the zero value of the Model that we can return along side an error, or `false`. Again, we use the `s.new` callback to create the Model for scanning.

## Implementing a model

We have our Store implemented, and a Model interface. Now, let's implement a simple Model that can utilise the Store,

    func Scan(desttab map[string]any, fields []string, scan ScanFunc) error {
        dest := make([]any, 0, len(fields))

        for _, fld := range fields {
            if p, ok := desttab[fld]; ok {
                dest = append(dest, p)
            }
        }
        return scan(dest...)
    }

    type User struct {
        ID        int64
        Email     string
        Username  string
        Password  []byte
        CreatedAt time.Time
    }

    func (u *User) Primary() (string, any) {
        return "id", u.ID
    }

    func (u *User) Scan(fields []string, scan ScanFunc) error {
        return Scan(map[string]any{
            "id":         &u.ID,
            "email":      &u.Email,
            "username":   &u.Username,
            "password":   &u.Password,
            "created_at": &u.CreatedAt,
        }, fields, scan)
    }

    func (u *User) Params() map[string]any {
        return map[string]any{
            "email":      u.Email,
            "username":   u.Username,
            "password":   u.Password,
            "created_at": u.CreatedAt,
        }
    }

you'll notice that with the above implementation we invoke a function called `Scan` within the implementation of the `User.Scan` method. This is a utility method that can be used across multiple Model implementations for scanning in fields. This will ensure that only the fields we are given will be scanned, provided that they exist in the given `desttab`.

With the above Model implementation we can now create a Store for it like
so,

    users := NewStore[*User](pool, "users", func() *User {
        return &User{}
    })

then, we can use the Store too,

    err := users.Create(ctx, &User{
        Email:     "gordon.freeman@blackmesa.com",
        Username:  "gordon.freeman",
        Password:  []byte("secret"),
        CreatedAt: time.Now(),
    })

    if err != nil {
        // Handle the error.
    }

    fmt.Println("user created with id of", u.ID)

    u, ok, err := users.Get(ctx, query.Where("username", "=", query.Arg("gordon.freeman")))

    if err != nil {
        // Handle the error.
    }

    if !ok {
        fmt.Println("user not found")
    }

    u.Password = []byte("unforseenconsequences")

    if err := users.Update(ctx, u); err != nil {
        // Handle the error.
    }

## Conclusion

I know that I dove in pretty quickly with this. What I wanted to explore was the possibility of creating an ORM-like experience in Go with generics. I wanted to implement something that was simple and extensible, and that did not rely on direct reflection to achieve what an ORM typically does, whilst making minimal assumptions about the data being modelled.

The above code can be found at the following GitHub [Gist][0], licensed under MIT, feel free to drop it into your code base and modify as you see fit. This won't be published as a conventional library, since it is less than 200 lines of code, and does not warrant being a library in my opinion. Furthermore, as I have stated, I think an ORM, if you could call this that, should make as few assumptions about the data being worked with as possible. Allowing you, the developer, to drop this into your code base, opens it up for extending even further according to your needs.

[0]: https://gist.github.com/andrewpillar/9f834b76dd2ba37dda79dca4a9a137f7