diff --git a/app/main.go b/app/main.go
index 02d38c3..7df139f 100644
--- a/app/main.go
+++ b/app/main.go
@@ -29,7 +29,18 @@ type CertFilter struct {
 	IdsNot               *[]string    `filterfield:"field=id;operator=NOT_IN"`
 	CreatedAt_Lt         *time.Time   `filterfield:"field=created_at;operator=LT"`
 	Timestamps           *[]time.Time `filterfield:"field=created_at;operator=IN"`
-	// CompanyIsActive      *bool        `filterfield:"joins=companies;field=is_active;operator=EQ"`
+	CompanyIsActive      *bool
+}
+
+func (f CertFilter) ApplyQuery(query *gorm.DB) *gorm.DB {
+	if f.CompanyIsActive != nil {
+		query = query.Joins(
+			fmt.Sprintf(
+				"JOIN companies ON certificates.company_id = companies.id WHERE companies.is_active = %t",
+				*f.CompanyIsActive,
+			))
+	}
+	return query
 }
 
 type CompanyFilter struct {
@@ -99,7 +110,7 @@ func doListWithJoins(db *gorm.DB) {
 	repo.Init(db, nil)
 
 	filter := CertFilter{
-		Alive: &TRUE,
+		CompanyIsActive: &FALSE,
 	}
 
 	certs, err := repo.List(filter, nil)
@@ -203,11 +214,11 @@ func main() {
 	db := db.InitDB()
 
 	// doMagic(db)
-	doList(db)
-	// doListWithJoins(db)
+	// doList(db)
+	doListWithJoins(db)
 	// doCount(db)
 	// doSave(db)
-	doDelete(db)
+	// doDelete(db)
 	// doGet(db)
 	// doExists(db)
 	// inheritance.DoInheritanceTest()
diff --git a/app/repository/smartfilter/filterfield.go b/app/repository/smartfilter/filterfield.go
index 8cc2c06..a4cf4b8 100644
--- a/app/repository/smartfilter/filterfield.go
+++ b/app/repository/smartfilter/filterfield.go
@@ -11,7 +11,6 @@ import (
 type FilterField struct {
 	Name     string
 	Operator Operator
-	Joins    []string
 
 	valueKind   reflect.Kind
 	boolValue   *bool
diff --git a/app/repository/smartfilter/smartfilter.go b/app/repository/smartfilter/smartfilter.go
index 84bd8d9..606e821 100644
--- a/app/repository/smartfilter/smartfilter.go
+++ b/app/repository/smartfilter/smartfilter.go
@@ -12,10 +12,15 @@ import (
 
 const TAG_NAME = "filterfield"
 const TAG_PAIRS_SEPARATOR = ";"
+const TAG_LIST_SEPARATOR = ","
 const TAG_KEYVALUE_SEPARATOR = "="
 
 type handlerFunc func(query *gorm.DB, tableName string, filterField *FilterField) *gorm.DB
 
+type QueryApplier interface {
+	ApplyQuery(query *gorm.DB) *gorm.DB
+}
+
 var operatorHandlers = map[Operator]handlerFunc{
 	OperatorEQ:     handleOperatorEQ,
 	OperatorNE:     handleOperatorNE,
@@ -67,15 +72,20 @@ func getFilterFields(filter interface{}) []ReflectedStructField {
 	return res
 }
 
+func getQueryApplierInterface(filter interface{}) QueryApplier {
+	queryApplier, ok := filter.(QueryApplier)
+	if ok {
+		return queryApplier
+	}
+	return nil
+}
+
 func ToQuery(model schema.Tabler, filter interface{}, query *gorm.DB) (*gorm.DB, error) {
 	st := reflect.TypeOf(filter)
 
 	tableName := model.TableName()
 	modelName := st.Name()
 
-	fmt.Printf("Table name: %s\n", tableName)
-	fmt.Printf("Model name: %s\n", modelName)
-
 	fields := getFilterFields(filter)
 	for _, field := range fields {
 		filterField, err := newFilterField(field.tagValue)
@@ -97,22 +107,36 @@ func ToQuery(model schema.Tabler, filter interface{}, query *gorm.DB) (*gorm.DB,
 		}
 	}
 
+	// apply custom filters, if interface exists
+	queryApplier := getQueryApplierInterface(filter)
+	if queryApplier != nil {
+		query = queryApplier.ApplyQuery(query)
+	}
+
 	return query, nil
 }
 
+func splitTrim(value string, separator string) []string {
+	var out []string = []string{}
+	for _, s := range strings.Split(value, separator) {
+		if len(s) == 0 {
+			continue
+		}
+		out = append(out, strings.TrimSpace(s))
+	}
+	return out
+}
+
 func newFilterField(tagValue string) (*FilterField, error) {
 	filterField := FilterField{}
 
-	tagValue = strings.TrimSpace(tagValue)
-	pairs := strings.Split(tagValue, TAG_PAIRS_SEPARATOR)
-
-	for _, pair := range pairs {
-		kvs := strings.Split(pair, TAG_KEYVALUE_SEPARATOR)
+	for _, pair := range splitTrim(tagValue, TAG_PAIRS_SEPARATOR) {
+		kvs := splitTrim(pair, TAG_KEYVALUE_SEPARATOR)
 		if len(kvs) != 2 {
 			return nil, fmt.Errorf("invalid tag value: %s", strings.TrimSpace(pair))
 		}
-		key := strings.TrimSpace(kvs[0])
-		value := strings.TrimSpace(kvs[1])
+		key := kvs[0]
+		value := kvs[1]
 
 		switch key {
 		case "field":
diff --git a/app/repository/smartfilter/smartfilter_test.go b/app/repository/smartfilter/smartfilter_test.go
index c9834d6..554e4a2 100644
--- a/app/repository/smartfilter/smartfilter_test.go
+++ b/app/repository/smartfilter/smartfilter_test.go
@@ -6,6 +6,7 @@ import (
 	"time"
 
 	"github.com/stretchr/testify/assert"
+	"gorm.io/gorm"
 )
 
 func TestGetFilterFields(t *testing.T) {
@@ -203,3 +204,32 @@ func TestFilterField(t *testing.T) {
 		assert.EqualError(t, err, "missing operator in tag: field=field_1")
 	})
 }
+
+type filterWithoutQueryApplier struct{}
+
+type filterWithQueryApplier struct{}
+
+func (f filterWithQueryApplier) ApplyQuery(query *gorm.DB) *gorm.DB {
+	return query
+}
+
+func TestSmartfilterApplyQuery(t *testing.T) {
+
+	t.Run("Get query applier interface - without interface", func(t *testing.T) {
+		f := filterWithoutQueryApplier{}
+		queryApplier := getQueryApplierInterface(f)
+		assert.Nil(t, queryApplier)
+	})
+
+	t.Run("Get query applier interface - with interface", func(t *testing.T) {
+		f := filterWithQueryApplier{}
+		queryApplier := getQueryApplierInterface(f)
+		assert.NotNil(t, queryApplier)
+	})
+
+	// t.Run("Get query applier interface - call interface function", func(t *testing.T) {
+	// 	f := filterWithQueryApplier{}
+	// 	queryApplier := getQueryApplierInterface(f)
+	// 	assert.NotNil(t, queryApplier)
+	// })
+}