Add first implementation of the Update operation

postgres.go

@@ -3,6 +3,7 @@ package gpostgres
 import (
 	"context"
 	"fmt"
+	"reflect"
 
 	"github.com/jinzhu/gorm"
 )
@@ -80,3 +81,86 @@ func (c Client) Delete(
 
 	return nil
 }
+
+// Update updates the given instances on the database by id.
+//
+// Partial updates are supported, i.e. it will ignore nil pointer attributes
+func (c Client) Update(
+	ctx context.Context,
+	items ...interface{},
+) error {
+	for _, item := range items {
+		m, err := structToMap(item)
+		if err != nil {
+			return err
+		}
+
+		r := c.db.Table(c.tableName).Updates(m)
+		if r.Error != nil {
+			return r.Error
+		}
+	}
+
+	return nil
+}
+
+// This cache is kept as a pkg variable
+// because the total number of types on a program
+// should be finite. So keeping a single cache here
+// works fine.
+var tagNamesCache = map[reflect.Type]map[int]string{}
+
+// structToMap converts any type to a map based on the
+// tag named `sql`, i.e. `sql:"map_key_name"`
+//
+// This function is efficient in the fact that it caches
+// the slower steps of the reflection required to do perform
+// this task.
+func structToMap(obj interface{}) (map[string]interface{}, error) {
+	v := reflect.ValueOf(obj)
+	t := v.Type()
+
+	if t.Kind() == reflect.Ptr {
+		t = t.Elem()
+		v = v.Elem()
+	}
+	if t.Kind() != reflect.Struct {
+		return nil, fmt.Errorf("input must be a struct or struct pointer")
+	}
+
+	names, found := tagNamesCache[t]
+	if !found {
+		names = getTagNames(t)
+		tagNamesCache[t] = names
+	}
+
+	m := map[string]interface{}{}
+	for i := 0; i < v.NumField(); i++ {
+		field := v.Field(i)
+		ft := field.Type()
+		if ft.Kind() == reflect.Ptr && field.IsNil() {
+			continue
+		}
+		m[names[i]] = field.Interface()
+	}
+
+	return m, nil
+}
+
+// This function collects only the names
+// that will be used from the input type.
+//
+// This should save several calls to `Field(i).Tag.Get("foo")`
+// which improves performance by a lot.
+func getTagNames(t reflect.Type) map[int]string {
+	resp := map[int]string{}
+	for i := 0; i < t.NumField(); i++ {
+		name := t.Field(i).Tag.Get("sql")
+		if name == "" {
+			continue
+		}
+		resp[i] = name
+	}
+
+	return resp
+}