diff --git a/examples/basic/server.ts b/examples/basic/server.ts
index 8d2e634..537ba93 100644
--- a/examples/basic/server.ts
+++ b/examples/basic/server.ts
@@ -19,6 +19,7 @@ const Piano = resource(v.object(
 		generationStrategy: autoIncrement,
 		serialize: (id) => id?.toString() ?? '0',
 		deserialize: (id) => Number.isFinite(Number(id)) ? Number(id) : 0,
+		schema: v.number(),
 	})
 	.canFetchItem()
 	.canFetchCollection()
@@ -43,6 +44,7 @@ const User = resource(v.object(
 		generationStrategy: autoIncrement,
 		serialize: (id) => id?.toString() ?? '0',
 		deserialize: (id) => Number.isFinite(Number(id)) ? Number(id) : 0,
+		schema: v.number(),
 	});
 
 const app = application({
diff --git a/src/core.ts b/src/core.ts
index 20fa2f1..70d784d 100644
--- a/src/core.ts
+++ b/src/core.ts
@@ -2,7 +2,7 @@ import * as http from 'http';
 import * as https from 'https';
 import { constants } from 'http2';
 import { pluralize } from 'inflection';
-import { BaseSchema, ObjectSchema } from 'valibot';
+import {BaseSchema, ObjectSchema, Output} from 'valibot';
 import { SerializerPair } from './serializers';
 import {
 	handleCreateItem, handleDeleteItem, handleEmplaceItem,
@@ -18,18 +18,16 @@ import * as en from './languages/en';
 import * as utf8 from './encodings/utf-8';
 import * as applicationJson from './serializers/application/json';
 
-// TODO define ResourceState
 // TODO separate frontend and backend factory methods
-// TODO complete content negotiation and default (fallback) messages collection
 
 export interface DataSource<T = object> {
 	initialize(): Promise<unknown>;
 	getTotalCount?(): Promise<number>;
 	getMultiple(): Promise<T[]>;
 	getSingle(id: string): Promise<T | null>;
-	create(data: Partial<T>): Promise<T>;
+	create(data: T): Promise<T>;
 	delete(id: string): Promise<unknown>;
-	emplace(id: string, data: Partial<T>): Promise<[T, boolean]>;
+	emplace(id: string, data: T): Promise<[T, boolean]>;
 	patch(id: string, data: Partial<T>): Promise<T | null>;
 }
 
@@ -38,24 +36,26 @@ export interface ApplicationParams {
 	dataSource?: (resource: Resource) => DataSource;
 }
 
-export interface Resource<T extends BaseSchema = any> {
+interface ResourceState<T extends BaseSchema> {
+	idAttr: string;
+	itemName: string;
+	collectionName: string;
+	routeName: string;
+	idConfig: ResourceIdConfig<T>;
+	fullTextAttrs: Set<string>;
+	canCreate: boolean;
+	canFetchCollection: boolean;
+	canFetchItem: boolean;
+	canPatch: boolean;
+	canEmplace: boolean;
+	canDelete: boolean;
+}
+
+export interface Resource<T extends BaseSchema = any, IdSchema extends BaseSchema = any> {
 	newId(dataSource: DataSource): string | number | unknown;
 	schema: T;
-	state: {
-		idAttr: string;
-		itemName?: string;
-		collectionName?: string;
-		routeName?: string;
-		idSerializer: NonNullable<IdParams['serialize']>;
-		idDeserializer: NonNullable<IdParams['deserialize']>;
-		canCreate: boolean;
-		canFetchCollection: boolean;
-		canFetchItem: boolean;
-		canPatch: boolean;
-		canEmplace: boolean;
-		canDelete: boolean;
-	};
-	id(newIdAttr: string, params: IdParams): this;
+	state: ResourceState<IdSchema>;
+	id(newIdAttr: string, params: ResourceIdConfig<IdSchema>): this;
 	fullText(fullTextAttr: string): this;
 	name(n: string): this;
 	collection(n: string): this;
@@ -76,10 +76,11 @@ interface GenerationStrategy {
 	(dataSource: DataSource, ...args: unknown[]): Promise<string | number | unknown>;
 }
 
-interface IdParams {
+interface ResourceIdConfig<T extends BaseSchema> {
 	generationStrategy: GenerationStrategy;
-	serialize?: (id: unknown) => string;
-	deserialize?: (id: string) => unknown;
+	serialize: (id: unknown) => string;
+	deserialize: (id: string) => Output<T>;
+	schema: T;
 }
 
 const getAllowedMiddlewares = (resource: Resource, mainResourceId: string) => {
@@ -110,24 +111,7 @@ const getAllowedMiddlewares = (resource: Resource, mainResourceId: string) => {
 	return middlewares;
 };
 
-interface ResourceState {
-	idAttr: string
-	itemName: string
-	collectionName: string
-	routeName: string
-	idGenerationStrategy: GenerationStrategy
-	idSerializer: IdParams['serialize']
-	idDeserializer: IdParams['deserialize']
-	fullTextAttrs: Set<string>;
-	canCreate: boolean;
-	canFetchCollection: boolean;
-	canFetchItem: boolean;
-	canPatch: boolean;
-	canEmplace: boolean;
-	canDelete: boolean;
-}
-
-export const resource = <T extends BaseSchema>(schema: T): Resource<T> => {
+export const resource = <T extends BaseSchema, Id extends BaseSchema = any>(schema: T): Resource<T, Id> => {
 	const resourceState = {
 		fullTextAttrs: new Set<string>(),
 		canCreate: false,
@@ -136,13 +120,13 @@ export const resource = <T extends BaseSchema>(schema: T): Resource<T> => {
 		canPatch: false,
 		canEmplace: false,
 		canDelete: false,
-	} as Partial<ResourceState>;
+	} as Partial<ResourceState<Id>>;
 
 	return {
-		get state(): ResourceState {
+		get state(): ResourceState<Id> {
 			return Object.freeze({
 				...resourceState
-			}) as unknown as ResourceState;
+			}) as unknown as ResourceState<Id>;
 		},
 		canFetchCollection(b = true) {
 			resourceState.canFetchCollection = b;
@@ -168,15 +152,13 @@ export const resource = <T extends BaseSchema>(schema: T): Resource<T> => {
 			resourceState.canDelete = b;
 			return this;
 		},
-		id(newIdAttr: string, params: IdParams) {
+		id(newIdAttr: string, params: ResourceIdConfig<Id>) {
 			resourceState.idAttr = newIdAttr;
-			resourceState.idGenerationStrategy = params.generationStrategy;
-			resourceState.idSerializer = params.serialize;
-			resourceState.idDeserializer = params.deserialize;
+			resourceState.idConfig = params;
 			return this;
 		},
 		newId(dataSource: DataSource) {
-			return resourceState?.idGenerationStrategy?.(dataSource);
+			return resourceState?.idConfig?.generationStrategy?.(dataSource);
 		},
 		fullText(fullTextAttr: string) {
 			if (
@@ -363,7 +345,7 @@ export const application = (appParams: ApplicationParams): Application => {
 		encodings: new Map<string, EncodingPair>(),
 	};
 
-	appState.languages.set(en.code, en.messages);
+	appState.languages.set(en.name, en.messages);
 	appState.encodings.set(utf8.name, utf8);
 	appState.serializers.set(applicationJson.name, applicationJson);
 
@@ -393,7 +375,7 @@ export const application = (appParams: ApplicationParams): Application => {
 			const clientState = {
 				contentType: applicationJson.name,
 				encoding: utf8.name,
-				language: en.code
+				language: en.name
 			};
 
 			return {
@@ -414,7 +396,7 @@ export const application = (appParams: ApplicationParams): Application => {
 		createBackend(): Backend {
 			const backendState: BackendState = {
 				fallback: {
-					language: en.code,
+					language: en.name,
 					encoding: utf8.name,
 					serializer: applicationJson.name
 				},
diff --git a/src/data-sources/file-jsonl.ts b/src/data-sources/file-jsonl.ts
index b94a2ea..26dd119 100644
--- a/src/data-sources/file-jsonl.ts
+++ b/src/data-sources/file-jsonl.ts
@@ -29,8 +29,9 @@ export class DataSource<T extends Record<string, string>> implements DataSourceI
 		return [...this.data];
 	}
 
-	async getSingle(id: string) {
-		const foundData = this.data.find((s) => this.resource.state.idSerializer(s[this.resource.state.idAttr as string]) === id);
+	async getSingle(idSerialized: string) {
+		const id = this.resource.state.idConfig.deserialize(idSerialized);
+		const foundData = this.data.find((s) => s[this.resource.state.idAttr as string] === id);
 
 		if (foundData) {
 			return {
@@ -41,13 +42,13 @@ export class DataSource<T extends Record<string, string>> implements DataSourceI
 		return null;
 	}
 
-	async create(data: Partial<T>) {
+	async create(data: T) {
 		const newData = {
 			...data
 		} as Record<string, unknown>;
 
 		if (this.resource.state.idAttr in newData) {
-			newData[this.resource.state.idAttr] = this.resource.state.idDeserializer(newData[this.resource.state.idAttr] as string);
+			newData[this.resource.state.idAttr] = this.resource.state.idConfig.deserialize(newData[this.resource.state.idAttr] as string);
 		}
 
 		const newCollection = [
@@ -60,26 +61,29 @@ export class DataSource<T extends Record<string, string>> implements DataSourceI
 		return data as T;
 	}
 
-	async delete(id: string) {
+	async delete(idSerialized: string) {
 		const oldDataLength = this.data.length;
 
-		const newData = this.data.filter((s) => !(this.resource.state.idSerializer(s[this.resource.state.idAttr as string]) === id));
+		const id = this.resource.state.idConfig.deserialize(idSerialized);
+		const newData = this.data.filter((s) => !(s[this.resource.state.idAttr] === id));
 
 		await writeFile(this.path, newData.map((d) => JSON.stringify(d)).join('\n'));
 
 		return oldDataLength !== newData.length;
 	}
 
-	async emplace(id: string, data: Partial<T>) {
-		const existing = await this.getSingle(id);
+	async emplace(idSerialized: string, dataWithId: T) {
+		const existing = await this.getSingle(idSerialized);
+		const id = this.resource.state.idConfig.deserialize(idSerialized);
+		const { [this.resource.state.idAttr]: idFromResource, ...data } = dataWithId;
 		const dataToEmplace = {
 			...data,
-			[this.resource.state.idAttr]: this.resource.state.idDeserializer(id),
-		};
+			[this.resource.state.idAttr]: id,
+		} as T;
 
 		if (existing) {
 			const newData = this.data.map((d) => {
-				if (this.resource.state.idSerializer(d[this.resource.state.idAttr as string]) === id) {
+				if (d[this.resource.state.idAttr] === id) {
 					return dataToEmplace;
 				}
 
@@ -88,16 +92,15 @@ export class DataSource<T extends Record<string, string>> implements DataSourceI
 
 			await writeFile(this.path, newData.map((d) => JSON.stringify(d)).join('\n'));
 
-			return [data, false] as [T, boolean];
+			return [dataToEmplace, false] as [T, boolean];
 		}
 
 		const newData = await this.create(dataToEmplace);
 		return [newData, true] as [T, boolean];
 	}
 
-	async patch(id: string, data: Partial<T>) {
-		const existing = await this.getSingle(id);
-
+	async patch(idSerialized: string, data: Partial<T>) {
+		const existing = await this.getSingle(idSerialized);
 		if (!existing) {
 			return null;
 		}
@@ -107,8 +110,9 @@ export class DataSource<T extends Record<string, string>> implements DataSourceI
 			...data,
 		}
 
+		const id = this.resource.state.idConfig.deserialize(idSerialized);
 		const newData = this.data.map((d) => {
-			if (this.resource.state.idSerializer(d[this.resource.state.idAttr as string]) === id) {
+			if (d[this.resource.state.idAttr as string] === id) {
 				return newItem;
 			}
 
@@ -116,7 +120,6 @@ export class DataSource<T extends Record<string, string>> implements DataSourceI
 		});
 
 		await writeFile(this.path, newData.map((d) => JSON.stringify(d)).join('\n'));
-
 		return newItem as T;
 	}
 }
diff --git a/src/handlers.ts b/src/handlers.ts
index 26355ab..6453ce4 100644
--- a/src/handlers.ts
+++ b/src/handlers.ts
@@ -667,7 +667,7 @@ export const handleEmplaceItem: Middleware = ({
 					v.object({
 						[resource.state.idAttr]: v.transform(
 							v.any(),
-							input => resource.state.idSerializer(input),
+							input => resource.state.idConfig.serialize(input),
 							v.literal(resourceId)
 						)
 					})
@@ -723,6 +723,7 @@ export const handleEmplaceItem: Middleware = ({
 	try {
 		// TODO error handling for each process
 		const params = bodyDeserialized as Record<string, unknown>;
+		params[resource.state.idAttr] = resource.state.idConfig.deserialize(params[resource.state.idAttr] as string);
 		const [newObject, isCreated] = await resource.dataSource.emplace(resourceId, params);
 		const serialized = responseBodySerializerPair.serialize(newObject);
 		const theFormatted = encoding.encode(serialized);
diff --git a/src/languages/en/index.ts b/src/languages/en/index.ts
index dc13506..a315097 100644
--- a/src/languages/en/index.ts
+++ b/src/languages/en/index.ts
@@ -94,4 +94,4 @@ export const messages: MessageCollection = {
 	}
 };
 
-export const code = 'en';
+export const name = 'en';
diff --git a/test/e2e/default.test.ts b/test/e2e/default.test.ts
index f26b2d2..f25c4b0 100644
--- a/test/e2e/default.test.ts
+++ b/test/e2e/default.test.ts
@@ -83,6 +83,7 @@ describe('yasumi', () => {
 				generationStrategy: autoIncrement,
 				serialize: (id) => id?.toString() ?? '0',
 				deserialize: (id) => Number.isFinite(Number(id)) ? Number(id) : 0,
+				schema: v.number(),
 			})
 	});
 
@@ -300,7 +301,6 @@ describe('yasumi', () => {
 			Piano.canCreate(false);
 		});
 
-		// FIXME ID de/serialization problems
 		it('returns data', () => {
 			return new Promise<void>((resolve, reject) => {
 				const req = request(
@@ -480,7 +480,6 @@ describe('yasumi', () => {
 			Piano.canEmplace(false);
 		});
 
-		// FIXME IDs not properly being de/serialized
 		it('returns data for replacement', () => {
 			return new Promise<void>((resolve, reject) => {
 				const req = request(