1 files changed, 74 insertions, 0 deletions
diff --git a/entity/resolver.go b/entity/resolver.go
new file mode 100644
index 00000000..d4fe5d3e
--- /dev/null
+++ b/entity/resolver.go
@@ -0,0 +1,74 @@
+package entity
+
+import (
+	"fmt"
+	"sync"
+)
+
+// Resolver is an interface to find an Entity from its Id
+type Resolver interface {
+	Resolve(id Id) (Interface, error)
+}
+
+// Resolvers is a collection of Resolver, for different type of Entity
+type Resolvers map[Interface]Resolver
+
+// Resolve use the appropriate sub-resolver for the given type and find the Entity matching the Id.
+func Resolve[T Interface](rs Resolvers, id Id) (T, error) {
+	var zero T
+	for t, resolver := range rs {
+		switch t.(type) {
+		case T:
+			val, err := resolver.(Resolver).Resolve(id)
+			if err != nil {
+				return zero, err
+			}
+			return val.(T), nil
+		}
+	}
+	return zero, fmt.Errorf("unknown type to resolve")
+}
+
+var _ Resolver = &CachedResolver{}
+
+// CachedResolver is a resolver ensuring that loading is done only once through another Resolver.
+type CachedResolver struct {
+	resolver Resolver
+	mu       sync.RWMutex
+	entities map[Id]Interface
+}
+
+func NewCachedResolver(resolver Resolver) *CachedResolver {
+	return &CachedResolver{
+		resolver: resolver,
+		entities: make(map[Id]Interface),
+	}
+}
+
+func (c *CachedResolver) Resolve(id Id) (Interface, error) {
+	c.mu.RLock()
+	if i, ok := c.entities[id]; ok {
+		c.mu.RUnlock()
+		return i, nil
+	}
+	c.mu.RUnlock()
+
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	i, err := c.resolver.Resolve(id)
+	if err != nil {
+		return nil, err
+	}
+	c.entities[id] = i
+	return i, nil
+}
+
+var _ Resolver = ResolverFunc(nil)
+
+// ResolverFunc is a helper to morph a function resolver into a Resolver
+type ResolverFunc func(id Id) (Interface, error)
+
+func (fn ResolverFunc) Resolve(id Id) (Interface, error) {
+	return fn(id)
+}