Fix: Deep partial stripping out fields in union

package.json

@@ -58,7 +58,8 @@
       "got@<11.8.5": ">=11.8.5",
       "semver": ">=7.5.2",
       "word-wrap": ">=1.2.4",
-      "braces": ">=3.0.3"
+      "braces": ">=3.0.3",
+      "micromatch": ">=4.0.8"
     }
   },
   "exports": {

src/zod-utils.ts

@@ -35,6 +35,57 @@ export const parseFieldsAsArrays = <T extends Record<string, unknown>>(
   ) as T
 }
 
+/**
+ * Merge all objects into a single one. If a property appears on multiple members, it
+ * will create a ZodUnion with all possible values for that property.
+ *
+ * This is important because, when dealing with a union of multiple objects where all
+ * fields are optional, zod won't be able to differentiate between them in a union, and
+ * will end up always choosing the first union item, even if it means stripping some fields.
+ * @param schemas
+ */
+export const mergeObjectSchemas = (schemas: z.ZodTypeAny[]): z.ZodTypeAny => {
+  const objects = schemas.filter(
+    (schema): schema is z.ZodObject<any> => schema instanceof z.ZodObject
+  )
+  // It could be a union of a ZodObject and a ZodNumber for example
+  const otherTypes = schemas.filter(
+    (schema) => !(schema instanceof z.ZodObject)
+  )
+
+  // Get all possible ZodTypes for each property
+  const mergedTypes = objects
+    .flatMap((obj) => Object.entries(obj.shape) as [string, ZodTypeAny][])
+    .reduce((acc, [key, value]): Record<string, ZodTypeAny[]> => {
+      if (key in acc) return { ...acc, [key]: [...acc[key], value] }
+      return { ...acc, [key]: [value] }
+    }, {} as Record<string, ZodTypeAny[]>)
+
+  const mergedShape = Object.fromEntries(
+    Object.entries(mergedTypes).map(([key, values]) => {
+      // This is not 100% because it will only dedupe by reference
+      const uniqueValues = Array.from(new Set(values))
+      const valueOrUnion =
+        uniqueValues.length === 1
+          ? uniqueValues[0]
+          : ZodUnion.create(
+              uniqueValues as [ZodTypeAny, ZodTypeAny, ...ZodTypeAny[]]
+            )
+      // If the value doesn't appear in all objects, make sure it's optional
+      const optionalValue =
+        values.length < objects.length
+          ? ZodOptional.create(valueOrUnion)
+          : valueOrUnion
+      return [key, optionalValue]
+    })
+  )
+
+  const allOptions = [...otherTypes, ZodObject.create(mergedShape)]
+  return allOptions.length === 1
+    ? allOptions[0]
+    : ZodUnion.create(allOptions as [ZodTypeAny, ZodTypeAny, ...ZodTypeAny[]])
+}
+
 // Copied from github.com/colinhacks/zod/blob/6dad90785398885f7b058f5c0760d5ae5476b833/src/types.ts#L2189-L2217
 // and extended to support unions and discriminated unions
 export const zodDeepPartial = (schema: ZodTypeAny): ZodTypeAny => {
@@ -57,9 +108,8 @@ export const zodDeepPartial = (schema: ZodTypeAny): ZodTypeAny => {
   } else if (schema instanceof ZodUnion) {
     type Options = [ZodTypeAny, ZodTypeAny, ...ZodTypeAny[]]
     const options = schema._def.options as Options
-    return ZodUnion.create(
-      options.map((option) => zodDeepPartial(option)) as Options
-    ) as any
+    const partialOptions = options.map(zodDeepPartial)
+    return mergeObjectSchemas(partialOptions)
   } else if (schema instanceof ZodDiscriminatedUnion) {
     const types = Object.values(schema.options) as (AnyZodObject &
       ZodRawShape)[]

test/zod-utils.test.ts

@@ -1,5 +1,52 @@
 import * as z from 'zod'
-import { zodDeepPartial, parseFieldsAsArrays } from '../src/zod-utils'
+import {
+  zodDeepPartial,
+  parseFieldsAsArrays,
+  mergeObjectSchemas,
+} from '../src/zod-utils'
+
+describe('mergeObjectSchemas', () => {
+  test('should keep the properties of aa single object', () => {
+    const schema = mergeObjectSchemas([
+      z.object({ n: z.number(), s: z.string().optional() }),
+    ])
+    expect(schema.parse({ n: 1 })).toEqual({ n: 1 })
+    expect(schema.parse({ n: 1, s: 'a' })).toEqual({ n: 1, s: 'a' })
+    expect(schema.parse({ n: 1, unrelated: 'a' })).toEqual({ n: 1 })
+    expect(() => schema.parse({ s: 'a' })).toThrow()
+  })
+  test('should merge the properties of multiple objects', () => {
+    const schema = mergeObjectSchemas([
+      z.object({ type: z.literal('a'), n: z.number() }),
+      z.object({ type: z.literal('b'), s: z.string() }),
+      z.object({ n: z.boolean().optional() }),
+    ])
+
+    expect(schema.parse({})).toEqual({})
+    expect(schema.parse({ n: 1 })).toEqual({ n: 1 })
+    expect(schema.parse({ n: true })).toEqual({ n: true })
+
+    expect(schema.parse({ type: 'a' })).toEqual({ type: 'a' })
+    expect(schema.parse({ type: 'b' })).toEqual({ type: 'b' })
+    expect(() => schema.parse({ type: 'c' })).toThrow()
+
+    expect(schema.parse({ s: 'b' })).toEqual({ s: 'b' })
+  })
+  test('should be compatible with non-object types', () => {
+    const schema = mergeObjectSchemas([
+      z.object({ type: z.literal('a'), n: z.number() }),
+      z.object({ type: z.literal('b'), s: z.string() }),
+      z.number(),
+      z.null(),
+    ])
+
+    expect(schema.parse(1)).toEqual(1)
+    expect(schema.parse(null)).toEqual(null)
+    expect(() => schema.parse({ type: 'c' })).toThrow()
+    expect(() => schema.parse({})).toThrow()
+    expect(() => schema.parse('str')).toThrow()
+  })
+})
 
 describe('zodDeepPartial', () => {
   test('partial of primitives', () => {
@@ -34,18 +81,59 @@ describe('zodDeepPartial', () => {
   })
 
   test('partial of unions', () => {
-    const deepPartial = zodDeepPartial(
-      z.union([
-        z.object({ s: z.string() }).strict(),
-        z.object({ n: z.number() }).strict(),
-      ])
-    )
+    const ZTestUnion = z.union([
+      z.object({ s: z.string() }).strict(),
+      z.object({ n: z.number() }).strict(),
+    ])
+    const deepPartial = zodDeepPartial(ZTestUnion)
 
     expect(deepPartial.parse({ s: 'string' })).toEqual({ s: 'string' })
     expect(deepPartial.parse({ n: 10 })).toEqual({ n: 10 })
     expect(deepPartial.parse({})).toEqual({})
 
     expect(() => deepPartial.parse({ n: 'string' })).toThrow()
+
+    const ZNestedTestUnion = z.object({
+      field: z.number(),
+      union: ZTestUnion,
+    })
+    const nestedDeepPartial = zodDeepPartial(ZNestedTestUnion)
+    expect(
+      nestedDeepPartial.parse({ field: 1, union: { s: 'string' } })
+    ).toEqual({ field: 1, union: { s: 'string' } })
+    expect(nestedDeepPartial.parse({ field: 1, union: { n: 10 } })).toEqual({
+      field: 1,
+      union: { n: 10 },
+    })
+    expect(nestedDeepPartial.parse({ field: 1, union: {} })).toEqual({
+      field: 1,
+      union: {},
+    })
+
+    const deepPartial2 = zodDeepPartial(
+      z.union([
+        z.object({
+          t: z.literal('a'),
+          s: z.string(),
+        }),
+        z.object({ t: z.literal('b'), n: z.number() }),
+      ])
+    )
+
+    expect(deepPartial2.parse({ s: 'a' })).toEqual({ s: 'a' })
+    expect(deepPartial2.parse({ n: 2 })).toEqual({ n: 2 })
+
+    const ZUnionOfUnion = z.union([
+      z.object({ randomField: z.string().optional() }),
+      ZTestUnion,
+    ])
+    const deepPartial3 = zodDeepPartial(ZUnionOfUnion)
+    expect(deepPartial3.parse({ s: 'a' })).toEqual({ s: 'a' })
+    expect(deepPartial3.parse({ n: 2 })).toEqual({ n: 2 })
+    expect(deepPartial3.parse({ randomField: 'r', n: 3 })).toEqual({
+      randomField: 'r',
+      n: 3,
+    })
   })
 
   test('partial of discriminated union', () => {