diff --git a/README.MD b/README.MD
index ac768e11..884beac1 100644
--- a/README.MD
+++ b/README.MD
@@ -53,7 +53,7 @@ read the [docs](https://rapiq.tada5hi.net).
 ### Build 🏗
 
 The first step is to construct a [BuildInput](https://rapiq.tada5hi.net/guide/build-api-reference.html#buildinput) object for a generic Record `<T>` and
-pass it to the [buildQuery](https://rapiq.tada5hi.net/guide/build-api-reference.html#buildquery) method to convert it to a string.
+pass it to the [buildQuery](https://rapiq.tada5hi.net/guide/build-api-reference.html#buildquery) method to convert it to a transportable string.
 
 The result string can then be provided as a URL query string to a backend application.
 The backend application can than process the request, by [parsing](#parse-) the query string.
diff --git a/src/parameter/fields/parse.ts b/src/parameter/fields/parse.ts
index 587b1b35..ed13513f 100644
--- a/src/parameter/fields/parse.ts
+++ b/src/parameter/fields/parse.ts
@@ -34,18 +34,6 @@ function buildReverseRecord(
     return output;
 }
 
-export function replaceRecordKey(record: Record<string, any>, key: string, newKey: string) : Record<string, any> {
-    if (
-        hasOwnProperty(record, key)
-    ) {
-        const value = record[key];
-        delete record[key];
-        record[newKey] = value;
-    }
-
-    return record;
-}
-
 export function parseQueryFields(
     data: unknown,
     options?: FieldsParseOptions,
diff --git a/src/parameter/fields/type.ts b/src/parameter/fields/type.ts
index 0e352f3f..264a6f6c 100644
--- a/src/parameter/fields/type.ts
+++ b/src/parameter/fields/type.ts
@@ -27,14 +27,15 @@ export type FieldsBuildInput<T extends Record<string, any>> =
                 never
         }
         |
-        [
-            FieldWithOperator<SimpleKeys<T>>[],
+        (
+            FieldWithOperator<SimpleKeys<T>>[]
+            |
             {
                 [K in keyof T]?: Flatten<T[K]> extends OnlyObject<T[K]> ?
                     FieldsBuildInput<Flatten<T[K]>> :
                     never
-            },
-        ]
+            }
+        )[]
         |
         FieldWithOperator<NestedKeys<T>>[] |
         FieldWithOperator<NestedKeys<T>>;
diff --git a/src/parameter/fields/utils/domain.ts b/src/parameter/fields/utils/domain.ts
index 8ce0aecc..e05cb16c 100644
--- a/src/parameter/fields/utils/domain.ts
+++ b/src/parameter/fields/utils/domain.ts
@@ -5,7 +5,6 @@
  * view the LICENSE file that was distributed with this source code.
  */
 
-import { hasOwnProperty } from '../../../utils';
 import { DEFAULT_ID } from '../../../constants';
 
 export function buildFieldDomainRecords(
@@ -25,41 +24,3 @@ export function buildFieldDomainRecords(
 
     return domainFields;
 }
-
-export function mergeFieldsDomainRecords(
-    sourceA: Record<string, string[]>,
-    sourceB: Record<string, string[]>,
-) {
-    const target: Record<string, string[]> = {};
-
-    let keys = Object.keys(sourceA);
-    for (let i = 0; i < keys.length; i++) {
-        if (hasOwnProperty(sourceB, keys[i])) {
-            target[keys[i]] = [...sourceA[keys[i]], ...sourceB[keys[i]]];
-        } else {
-            target[keys[i]] = [...sourceA[keys[i]]];
-        }
-    }
-
-    keys = Object.keys(sourceB).filter((key) => !keys.includes(key));
-    for (let i = 0; i < keys.length; i++) {
-        if (hasOwnProperty(sourceA, keys[i])) {
-            target[keys[i]] = [...sourceA[keys[i]], ...sourceB[keys[i]]];
-        } else {
-            target[keys[i]] = [...sourceB[keys[i]]];
-        }
-    }
-
-    keys = Object.keys(target);
-
-    if (
-        keys.length >= 2 &&
-        hasOwnProperty(target, DEFAULT_ID) &&
-        Array.isArray(target[DEFAULT_ID]) &&
-        target[DEFAULT_ID].length === 0
-    ) {
-        delete target[DEFAULT_ID];
-    }
-
-    return target;
-}
diff --git a/src/parameter/filters/build.ts b/src/parameter/filters/build.ts
index 8119cc7b..6e1332df 100644
--- a/src/parameter/filters/build.ts
+++ b/src/parameter/filters/build.ts
@@ -36,11 +36,23 @@ export function buildQueryFilters<T>(
                 return true;
             }
 
+            if (Array.isArray(input)) {
+                // todo: check array elements are string
+                output[key] = input.join(',');
+
+                return true;
+            }
+
             if (isFilterOperatorConfig(input)) {
                 if (typeof input.value === 'undefined') {
                     input.value = null;
                 }
 
+                if (Array.isArray(input.value)) {
+                    // todo: check array elements are string
+                    input.value = input.value.join(',');
+                }
+
                 if (Array.isArray(input.operator)) {
                     // merge operators
                     input.operator = input.operator
diff --git a/src/parameter/filters/type.ts b/src/parameter/filters/type.ts
index c3ea530b..76ed0c84 100644
--- a/src/parameter/filters/type.ts
+++ b/src/parameter/filters/type.ts
@@ -22,8 +22,8 @@ type FilterValueInput = FilterValueInputPrimitive | null | undefined;
 
 export type FilterValueSimple<V extends FilterValueInput = FilterValueInput> = V extends FilterValueInputPrimitive ? (V | V[]) : V;
 export type FilterValueWithOperator<V extends FilterValueInput = FilterValueInput> = V extends string | number ?
-    `!${V}` | `!~${V}` | `~${V}` | `<${V}` | `<=${V}` | `>${V}` | `>=${V}` :
-    never;
+    `!${V}` | `!~${V}` | `~${V}` | `<${V}` | `<=${V}` | `>${V}` | `>=${V}` | null | '!null' :
+    V extends boolean ? null | '!null' : never;
 
 export type FilterValue<V extends FilterValueInput = FilterValueInput> = V extends FilterValueInputPrimitive ?
     (FilterValueSimple<V> | FilterValueWithOperator<V> | Array<FilterValueWithOperator<V>>) :
diff --git a/src/parse/module.ts b/src/parse/module.ts
index 8ec5dde0..5810c6b9 100644
--- a/src/parse/module.ts
+++ b/src/parse/module.ts
@@ -8,9 +8,9 @@
 import {
     FieldsParseOutput,
     FiltersParseOutput,
-    PaginationParseOutput, RelationsParseOutput,
+    PaginationParseOutput,
+    RelationsParseOutput,
     SortParseOutput,
-    parseQueryRelations,
 } from '../parameter';
 import { Parameter, URLParameter } from '../constants';
 import { parseQueryParameter } from './parameter';
@@ -43,7 +43,8 @@ export function parseQuery(
             case Parameter.RELATIONS: {
                 const value = input[Parameter.RELATIONS] ?? input[URLParameter.RELATIONS];
                 if (value || options[Parameter.RELATIONS]) {
-                    relations = parseQueryRelations(
+                    relations = parseQueryParameter(
+                        key,
                         value,
                         options[Parameter.RELATIONS],
                     );
@@ -56,7 +57,7 @@ export function parseQuery(
                 const value = input[Parameter.FIELDS] ?? input[URLParameter.FIELDS];
                 if (value || options[Parameter.FIELDS]) {
                     output[Parameter.FIELDS] = parseQueryParameter(
-                        keys[i],
+                        key,
                         value,
                         options[Parameter.FIELDS],
                         relations,
@@ -68,7 +69,7 @@ export function parseQuery(
                 const value = input[Parameter.FILTERS] ?? input[URLParameter.FILTERS];
                 if (value || options[Parameter.FILTERS]) {
                     output[Parameter.FILTERS] = parseQueryParameter(
-                        keys[i],
+                        key,
                         value,
                         options[Parameter.FILTERS],
                         relations,
@@ -80,7 +81,7 @@ export function parseQuery(
                 const value = input[Parameter.PAGINATION] ?? input[URLParameter.PAGINATION];
                 if (value || options[Parameter.PAGINATION]) {
                     output[Parameter.PAGINATION] = parseQueryParameter(
-                        keys[i],
+                        key,
                         value,
                         options[Parameter.PAGINATION],
                         relations,
@@ -92,7 +93,7 @@ export function parseQuery(
                 const value = input[Parameter.SORT] ?? input[URLParameter.SORT];
                 if (value || options[Parameter.SORT]) {
                     output[Parameter.SORT] = parseQueryParameter(
-                        keys[i],
+                        key,
                         value,
                         options[Parameter.SORT],
                         relations,
diff --git a/test/unit/build.spec.ts b/test/unit/build.spec.ts
index 1d3fcd52..ba4ea513 100644
--- a/test/unit/build.spec.ts
+++ b/test/unit/build.spec.ts
@@ -65,6 +65,19 @@ describe('src/build.ts', () => {
         });
         expect(record).toEqual(buildURLQueryString({ [URLParameter.FILTERS]: { 'child.id': 1 } }));
 
+        record = buildQuery<Entity>({
+            filter: {
+                'child.id': 1,
+                child: {
+                    'child.id': 'abc'
+                },
+            },
+        });
+        expect(record).toEqual(buildURLQueryString({ [URLParameter.FILTERS]: {
+            'child.id': 1,
+            'child.child.id': 'abc'
+        } }));
+
         record = buildQuery<Entity>({
             filter: {
                 siblings: {
@@ -136,6 +149,14 @@ describe('src/build.ts', () => {
         expect(record).toEqual(buildURLQueryString({ [URLParameter.FILTERS]: { id: '>=1' } }));
 
         // with negation & in operator
+        record = buildQuery<Entity>({
+            filter: {
+                id: [1,2,3],
+            },
+        });
+        expect(record).toEqual(buildURLQueryString({ [URLParameter.FILTERS]: { id: '1,2,3' } }));
+
+        // with negation & like operator
         record = buildQuery<Entity>({
             filter: {
                 id: {
@@ -200,6 +221,32 @@ describe('src/build.ts', () => {
         });
 
         expect(record).toEqual(buildURLQueryString({ fields: { [DEFAULT_ID]: ['id'], child: ['id', 'name'] } }));
+
+        record = buildQuery<Entity>({
+            fields: [
+                ['id'],
+                ['name'],
+                {
+                    child: ['id', 'name'],
+                }
+            ]
+        });
+
+        expect(record).toEqual(buildURLQueryString({ fields: { [DEFAULT_ID]: ['id', 'name'], child: ['id', 'name'] } }));
+        record = buildQuery<Entity>({
+            fields: [
+                ['id'],
+                {
+                    child: ['id'],
+                },
+                {
+                    child: ['name'],
+                }
+            ]
+        });
+
+        expect(record).toEqual(buildURLQueryString({ fields: { [DEFAULT_ID]: ['id'], child: ['id', 'name'] } }));
+
     });
 
     it('should format sort record', () => {
diff --git a/test/unit/filters.spec.ts b/test/unit/filters.spec.ts
index 5b89ff5d..2d6a25dd 100644
--- a/test/unit/filters.spec.ts
+++ b/test/unit/filters.spec.ts
@@ -12,6 +12,7 @@ import {
     parseQueryFilters,
     parseQueryRelations,
 } from '../../src';
+import {isFilterOperatorConfig} from "../../src/parameter/filters/utils";
 
 describe('src/filter/index.ts', () => {
     it('should transform request filters', () => {
@@ -349,4 +350,27 @@ describe('src/filter/index.ts', () => {
             },
         ] as FiltersParseOutput);
     });
+
+    it('should determine filter operator config', () => {
+        let data = isFilterOperatorConfig({
+            value: 1,
+            operator: '<'
+        });
+
+        expect(data).toBeTruthy();
+
+        data = isFilterOperatorConfig({
+            value: 1,
+            operator: {}
+        })
+
+        expect(data).toBeFalsy();
+
+        data = isFilterOperatorConfig({
+            value: {},
+            operator: '<'
+        });
+
+        expect(data).toBeFalsy();
+    })
 });