count, err := db.NewSelect().
   Model(&[]*B{}).
   Relation("A", func(q *bun.SelectQuery) *bun.SelectQuery {
      return q.Where("name = ?", "someName")
   }).
   Count(ctx)

package main

import (
	"context"
	"database/sql"
	"fmt"

	embeddedpostgres "github.com/fergusstrange/embedded-postgres"
	"github.com/uptrace/bun"
	"github.com/uptrace/bun/dialect/pgdialect"
	"github.com/uptrace/bun/driver/pgdriver"
)

type Foo struct {
	Id   int64 `bun:",pk,autoincrement"`
	Name string
}

type Bar struct {
	Id    int64 `bun:",pk,autoincrement"`
	FooId int64
	Foo   *Foo `bun:"rel:belongs-to,join:foo_id=id"`
}

func main() {
	pgServer := embeddedpostgres.NewDatabase(
		embeddedpostgres.DefaultConfig().Logger(nil),
	)
	err := pgServer.Start()
	if err != nil {
		panic(err)
	}
	defer pgServer.Stop()

	ctx := context.Background()
	dsn := "postgres://postgres:postgres@localhost:5432/postgres?sslmode=disable"
	sqlDb := sql.OpenDB(pgdriver.NewConnector(pgdriver.WithDSN(dsn)))
	db := bun.NewDB(sqlDb, pgdialect.New())

	_, err = db.NewCreateTable().
		Model((*Foo)(nil)).
		Exec(ctx)
	if err != nil {
		panic(err)
	}
	_, err = db.NewCreateTable().
		Model((*Bar)(nil)).
		Exec(ctx)
	if err != nil {
		panic(err)
	}

	_, err = db.NewInsert().
		Model(&[]*Foo{
			{Name: "first"},
			{Name: "second"},
		}).
		Exec(ctx)
	if err != nil {
		panic(err)
	}
	_, err = db.NewInsert().
		Model(&[]*Bar{
			{FooId: 1},
			{FooId: 2},
		}).
		Exec(ctx)
	if err != nil {
		panic(err)
	}

	count, err := db.NewSelect().
		Model(&[]*Bar{}).
		Relation("Foo", func(q *bun.SelectQuery) *bun.SelectQuery {
			return q.Where("name = 'first'")
		}).
		Count(ctx)
	if err != nil {
		panic(err)
	}
	fmt.Println("Result with Count:", count)

	count, err = db.NewSelect().
		Model(&[]*Bar{}).
		Relation("Foo", func(q *bun.SelectQuery) *bun.SelectQuery {
			return q.Where("name = 'first'")
		}).
		ScanAndCount(ctx)
	if err != nil {
		panic(err)
	}
	fmt.Println("Result with ScanAndCount:", count)
}