pg

go-pg

Postgresql Support for Go. This module provides:

• Binding SQL statements to named arguments;
• Support for mapping go structures to SQL tables, and vice versa;
• Easy semantics for Insert, Delete, Update, Get and List operations;
• Bulk insert operations and transactions;
• Support for tracing and observability.

Documentation: https://pkg.go.dev/github.com/djthorpe/go-pg

Motivation

The package provides a simple way to interact with a Postgresql database from Go, to reduce the amount of boilerplate code required to interact with the database. The supported operations align with API calls POST, PUT, GET, DELETE and PATCH.

• Insert - Insert a row into a table, and return the inserted row (POST or PUT);
• Delete - Delete one or more rows from a table, and optionally return the deleted rows (DELETE);
• Update - Update one or more rows in a table, and optionally return the updated rows (PATCH);
• Get - Get a single row from a table (GET);
• List - Get a list of rows from a table (GET).

In order to support database operations on go types, those types need to implement one or more of the following interfaces:

Selector

type Selector interface {
  // Bind row selection variables, returning the SQL statement required for the operation
  // The operation can be Get, Update, Delete or List
  Select(*Bind, Op) (string, error)
}

A type which implements a Selector interface can be used to select rows from a table, for get, list, update and deleting operations.

Reader

type Reader interface {
  // Scan a row into the receiver
  Scan(Row) error
}

A type which implements a Reader interface can be used to translate SQL types to the type instance. If multiple rows are returned, then the Scan method is called repeatly until no more rows are returned.

type ListReader interface {
  // Scan a count of returned rows into the receiver
  ScanCount(Row) error
}

A type which implements a ListReader interface can be used to scan the count of rows returned.

Writer

type Writer interface {
  // Bind insert parameters, returning the SQL statement required for the insert
  Insert(*Bind) (string, error)

  // Bind update parameters
  Update(*Bind) error
}

A type which implements a Writer interface can be used to bind object instance variables to SQL parameters. An example of how to implement an API gateway using this package is shown below.

Database Server Connection Pool

You can create a connection pool to a database server using the pg.NewPool function:

import (
  pg "github.com/djthorpe/go-pg"
)

func main() {
  pool, err := pg.NewPool(ctx,
    pg.WithHostPort(host, port),
    pg.WithCredentials("postgres", "password"),
    pg.WithDatabase(name),
  )
  if err != nil {
      panic(err)
  }
  defer pool.Close()

  // ...
}

The options that can be passed to pg.NewPool are:

• WithCredentials(string,string) - Set connection pool username and password. If the database name is not set, then the username will be used as the default database name.
• WithDatabase(string) - Set the database name for the connection. If the user name is not set, then the database name will be used as the user name.
• WithAddr(string) - Set the address (host) or (host:port) for the connection
• WithHostPort(string, string) - Set the hostname and port for the connection. If the port is not set, then the default port 5432 will be used.
• WithSSLMode( string) - Set the SSL connection mode. Valid values are "disable", "allow", "prefer", "require", "verify-ca", "verify-full". See https://www.postgresql.org/docs/current/libpq-ssl.html for more information.
• pg.WithTrace(pg.TraceFn) - Set the trace function for the connection pool. The signature of the trace unction is func(ctx context.Context, sql string, args any, err error) and is called for every query executed by the connection pool.
• pg.WithBind(string,any) - Set the bind variable to a value the the lifetime of the connection.

Executing Statements

To simply execute a statement, use the Exec call:

  if err := pool.Exec(ctx, `CREATE TABLE test (id SERIAL PRIMARY KEY, name TEXT)`); err != nil {
    panic(err)
  }

You can use bind variables to bind named arguments to a statement using the With function. Within the statement, the following formats are replaced with bound values:

• ${"name"} - Replace with the value of the named argument "name", double-quoted string
• ${'name'} - Replace with the value of the named argument "name", single-quoted string
• ${name} - Replace with the value of the named argument "name", unquoted string
• $$ - Pass a literal dollar sign
• @name - Pass by bound variable parameter

For example,

  var name string
  // ...
  if err := pool.With("table", "test", "name", name).Exec(ctx, `INSERT INTO ${"table"} (name) VALUES (@name)`); err != nil {
    panic(err)
  }

This will re-use or create a new database connection from the connection, pool, bind the named arguments, replace the named arguments in the statement, and execute the statement.

Implementing Get

If you have a http handler which needs to get a row from a table, you can implement a Selector interface. For example,

type MyObject struct {
  Id int
  Name string
}

// Reader - bind to object
func (obj *MyObject) Scan(row pg.Row) error {
  return row.Scan(&obj.Id, &obj.Name)
}

// Selector - select rows from database
func (obj MyObject) Select(bind *pg.Bind, op pg.Op) (string, error) {
  switch op {
  case pg.Get:
    bind.Set("id", obj.Id)
    return `SELECT id, name FROM mytable WHERE id=@id`, nil
  }
}

// Select the row from the database
func main() {
  // ...
  var obj MyObject
  if err := conn.Get(ctx, &obj, MyObject{ Id: 1 }); err != nil {
    panic(err)
  }
  // ...
}

Implementing List

You may wish to use paging to list rows from a table. The List operation is used to list rows from a table, with offset and limit parameters. The http handler may look like this:

func ListHandler(w http.ResponseWriter, r *http.Request) {
  var conn pg.Conn

  // ....Set pool....

  // Get up to 10 rows
  var response MyList
  if err := conn.List(ctx, &response, MyListRequest{Offset: 0, Limit: 10}); err != nil {
    http.Error(w, err.Error(), http.StatusInternalServerError)
    return
  }

  // Write the row to the response - TODO: Add Content-Type header
  json.NewEncoder(w).Encode(response)
}

The implementation of MyList and MyListRequest may look like this:

type MyListRequest struct {
  Offset uint64
  Limit uint64
}

type MyList struct {
  Count uint64
  Names []string
}

// Reader - note this needs to be a pointer receiver
func (obj *MyList) Scan(row pg.Row) error {
  var name string
  if err := row.Scan(&name); err != nil {
    return err
  }
  obj = append(obj, row.String())
  return nil
}

// ListReader - optional interface to scan count of all rows
func (obj MyList) Scan(row pg.Row) error {
 return row.Scan(&obj.Count)
}

// Selector - select rows from database. Use bind variables
// offsetlimit, groupby and orderby to filter the selected rows.
func (obj MyListRequest) Select(bind *pg.Bind, op pg.Op) (string, error) {
  bind.Set("offsetlimit", fmt.Sprintf("OFFSET %v LIMIT %v",obj.Offset,obj.Limit))
  switch op {
  case pg.List:
    return `SELECT name FROM mytable`, nil
  default:
    return "", fmt.Errorf("Unsupported operation: ",op)
  }
}

You can of course use a WHERE clause in your query to filter the rows returned from the table. Always implement the offsetlimit as a bind variable.

Implementing Insert

TODO

Implementing Patch

TODO

Implementing Delete

TODO

Transactions

Transactions are executed within a function called Tx. For example,

  if err := pool.Tx(ctx, func(tx pg.Tx) error {
    if err := tx.Exec(ctx, `CREATE TABLE test (id SERIAL PRIMARY KEY, name TEXT)`); err != nil {
      return err
    }
    if err := tx.Exec(ctx, `INSERT INTO test (name) VALUES ('hello')`); err != nil {
      return err
    }
    return nil
  }); err != nil {
    panic(err)
  }

Any error returned from the function will cause the transaction to be rolled back. If the function returns nil, then the transaction will be committed. Transactions can be nested.

Notify and Listen

TODO

Schema Support

• Checking if a schema exists
• Creating a schema
• Dropping a schema

Error Handing and Tracing

TODO

Testing Support

TODO