diff --git a/packages/orm/src/client/zod/factory.ts b/packages/orm/src/client/zod/factory.ts
index 53014d4b1..288403078 100644
--- a/packages/orm/src/client/zod/factory.ts
+++ b/packages/orm/src/client/zod/factory.ts
@@ -176,6 +176,19 @@ export class ZodSchemaFactory<
         return Object.entries(modelDef.fields).filter(([, def]) => def.type !== 'Unsupported');
     }
 
+    /**
+     * Maps each relation field of a model to the foreign-key fields it sets.
+     */
+    private getRelationForeignKeys(model: string): Record<string, readonly string[]> {
+        const result: Record<string, readonly string[]> = {};
+        for (const [name, fieldDef] of this.getModelFields(model)) {
+            if (fieldDef.relation?.fields?.length) {
+                result[name] = fieldDef.relation.fields;
+            }
+        }
+        return result;
+    }
+
     private shouldIncludeRelations(options?: CreateSchemaOptions): boolean {
         return options?.relationDepth === undefined || options.relationDepth > 0;
     }
@@ -2117,11 +2130,23 @@ export class ZodSchemaFactory<
             }
         });
 
+        // an update payload holds only the fields it changes
+        const partialOptions = { partial: true, relationForeignKeys: this.getRelationForeignKeys(model) };
         const uncheckedUpdateSchema = this.extraValidationsEnabled
-            ? ZodUtils.addCustomValidation(z.strictObject(uncheckedVariantFields), modelDef.attributes)
+            ? ZodUtils.addCustomValidation(
+                  z.strictObject(uncheckedVariantFields),
+                  modelDef.attributes,
+                  undefined,
+                  partialOptions,
+              )
             : z.strictObject(uncheckedVariantFields);
         const checkedUpdateSchema = this.extraValidationsEnabled
-            ? ZodUtils.addCustomValidation(z.strictObject(checkedVariantFields), modelDef.attributes)
+            ? ZodUtils.addCustomValidation(
+                  z.strictObject(checkedVariantFields),
+                  modelDef.attributes,
+                  undefined,
+                  partialOptions,
+              )
             : z.strictObject(checkedVariantFields);
         const result = !hasRelation ? uncheckedUpdateSchema : z.union([uncheckedUpdateSchema, checkedUpdateSchema]);
 
diff --git a/packages/zod/src/utils.ts b/packages/zod/src/utils.ts
index db773096e..a8309d292 100644
--- a/packages/zod/src/utils.ts
+++ b/packages/zod/src/utils.ts
@@ -307,8 +307,8 @@ export function addListValidation(
 }
 
 /**
- * Recursively collects all field names referenced by `kind: 'field'` nodes
- * inside an expression tree.
+ * Recursively collects all field names referenced by `kind: 'field'` nodes and
+ * by `this.field` member nodes inside an expression tree.
  */
 export function collectFieldRefs(expr: Expression): Set<string> {
     const refs = new Set<string>();
@@ -331,7 +331,12 @@ export function collectFieldRefs(expr: Expression): Set<string> {
                 e.items.forEach(walk);
                 break;
             case 'member':
-                walk(e.receiver);
+                if (e.receiver.kind === 'this') {
+                    // `this.field` references `field`
+                    if (e.members[0]) refs.add(e.members[0]);
+                } else {
+                    walk(e.receiver);
+                }
                 break;
             // literal / null / this / binding — no field refs
         }
@@ -351,6 +356,10 @@ export function collectFieldRefs(expr: Expression): Set<string> {
  * Omit `presentFields` (or pass `undefined`) to apply all rules
  * unconditionally, which is the correct behaviour for full-model schemas.
  *
+ * Pass `options.partial` when the data is an update payload, which holds only
+ * the fields it changes. A rule is then skipped for data that sends none of
+ * the fields it references: the update cannot change the rule's result.
+ *
  * Note: `@@validate` conditions are restricted by the ZModel compiler to
  * scalar fields of the same model only — relation fields are a compile error.
  * A flat field-name set is therefore sufficient.
@@ -359,6 +368,19 @@ export function addCustomValidation(
     schema: z.ZodSchema,
     attributes: readonly AttributeApplication[] | undefined,
     presentFields?: ReadonlySet<string>,
+    options?: {
+        /**
+         * The data holds only some of the model's fields, as in an update. A rule is then
+         * skipped when the data sends none of the fields it references.
+         */
+        partial?: boolean;
+        /**
+         * Maps each relation field to the foreign-key fields it sets. In partial mode, a
+         * relation field in the data counts as sending its foreign keys, unless it holds
+         * only a nested `update`, which cannot change them.
+         */
+        relationForeignKeys?: Record<string, readonly string[]>;
+    },
 ): z.ZodSchema {
     const attrs = attributes?.filter((a) => a.name === '@@validate');
     if (!attrs || attrs.length === 0) {
@@ -386,16 +408,71 @@ export function addCustomValidation(
         if (pathExpr && ExpressionUtils.isArray(pathExpr)) {
             path = pathExpr.items.map((e) => ExpressionUtils.getLiteralValue(e) as string);
         }
-        result = applyValidation(result, expr, message, path);
+        result = applyValidation(
+            result,
+            expr,
+            message,
+            path,
+            options?.partial ? partialTriggers(expr, options.relationForeignKeys) : undefined,
+        );
     }
     return result;
 }
 
+type PartialTriggers = {
+    /** The fields the rule references. */
+    fields: string[];
+    /** The relation fields that set a foreign key the rule references. */
+    relations: string[];
+};
+
+/**
+ * Returns the fields and relation fields that make a rule apply to partial data.
+ */
+function partialTriggers(
+    expr: Expression,
+    relationForeignKeys: Record<string, readonly string[]> = {},
+): PartialTriggers {
+    const refs = collectFieldRefs(expr);
+    return {
+        fields: [...refs],
+        relations: Object.entries(relationForeignKeys)
+            .filter(([, foreignKeys]) => foreignKeys.some((fk) => refs.has(fk)))
+            .map(([relation]) => relation),
+    };
+}
+
+/**
+ * Tells whether a relation payload can change the foreign keys of its relation. Every nested
+ * operation can, except `update`, which writes to the related row only.
+ */
+function relationChangesForeignKey(payload: unknown) {
+    return typeof payload === 'object' && payload !== null && Object.keys(payload).some((op) => op !== 'update');
+}
+
+/**
+ * Tells whether partial data sends a field or a foreign-key change that a rule depends on.
+ */
+function partialDataTriggersRule(data: unknown, triggers: PartialTriggers) {
+    if (typeof data !== 'object' || data === null) {
+        return false;
+    }
+    return (
+        triggers.fields.some((field) => field in data) ||
+        triggers.relations.some((relation) => relationChangesForeignKey((data as Record<string, unknown>)[relation]))
+    );
+}
+
+/**
+ * Adds one `@@validate` rule to `schema` as a refinement. With `triggers`, the data is partial and
+ * the refinement passes for data that does not trigger the rule.
+ */
 function applyValidation(
     schema: z.ZodSchema,
     expr: Expression,
     message: string | undefined,
     path: string[] | undefined,
+    triggers: PartialTriggers | undefined,
 ) {
     const options: Parameters<typeof schema.refine>[1] = {};
     if (message) {
@@ -404,7 +481,12 @@ function applyValidation(
     if (path) {
         options.path = path;
     }
-    return schema.refine((data) => Boolean(evalExpression(data, expr)), options);
+    return schema.refine((data) => {
+        if (triggers && !partialDataTriggersRule(data, triggers)) {
+            return true;
+        }
+        return Boolean(evalExpression(data, expr));
+    }, options);
 }
 
 function evalExpression(data: any, expr: Expression): unknown {
diff --git a/tests/regression/test/issue-2904.test.ts b/tests/regression/test/issue-2904.test.ts
new file mode 100644
index 000000000..100fd73c0
--- /dev/null
+++ b/tests/regression/test/issue-2904.test.ts
@@ -0,0 +1,150 @@
+import { createTestClient } from '@zenstackhq/testtools';
+import { describe, expect, it } from 'vitest';
+
+// https://github.com/zenstackhq/zenstack/issues/2904
+describe('Regression for issue 2904', () => {
+    const schema = `
+model User {
+    id   Int   @id @default(autoincrement())
+    bots Bot[]
+}
+
+model Bot {
+    id        Int     @id @default(autoincrement())
+    name      String
+    useCase   String?
+    modelName String?
+    owner     User?   @relation(fields: [ownerId], references: [id])
+    ownerId   Int?
+
+    @@validate(useCase != null || modelName != null, 'needs a use case or a model')
+}
+`;
+
+    it('accepts an update that sends none of the fields a rule references', async () => {
+        const db = await createTestClient(schema);
+        const user = await db.user.create({ data: {} });
+        const bot = await db.bot.create({ data: { name: 'a', useCase: 'chat', ownerId: user.id } });
+
+        await expect(db.bot.update({ where: { id: bot.id }, data: { name: 'b' } })).resolves.toMatchObject({
+            name: 'b',
+            useCase: 'chat',
+        });
+        await expect(db.bot.updateMany({ where: { id: bot.id }, data: { name: 'c' } })).resolves.toEqual({
+            count: 1,
+        });
+        await expect(
+            db.bot.upsert({
+                where: { id: bot.id },
+                create: { name: 'x', useCase: 'chat' },
+                update: { name: 'd' },
+            }),
+        ).resolves.toMatchObject({ name: 'd' });
+        await expect(
+            db.user.update({
+                where: { id: user.id },
+                data: { bots: { update: { where: { id: bot.id }, data: { name: 'e' } } } },
+                include: { bots: true },
+            }),
+        ).resolves.toMatchObject({ bots: [{ name: 'e' }] });
+    });
+
+    it('still checks a rule when the update sends a field it references', async () => {
+        const db = await createTestClient(schema);
+        const bot = await db.bot.create({ data: { name: 'a', useCase: 'chat' } });
+
+        await expect(
+            db.bot.update({ where: { id: bot.id }, data: { useCase: null, modelName: null } }),
+        ).toBeRejectedByValidation(['needs a use case or a model']);
+        await expect(
+            db.bot.update({ where: { id: bot.id }, data: { useCase: null, modelName: 'gpt' } }),
+        ).resolves.toMatchObject({ useCase: null, modelName: 'gpt' });
+    });
+
+    it('still checks every rule on create', async () => {
+        const db = await createTestClient(schema);
+        await expect(db.bot.create({ data: { name: 'a' } })).toBeRejectedByValidation(['needs a use case or a model']);
+    });
+
+    it('checks a rule when a relation field sets a foreign key the rule references', async () => {
+        const db = await createTestClient(`
+model User {
+    id    Int    @id @default(autoincrement())
+    posts Post[]
+}
+
+model Post {
+    id       Int    @id @default(autoincrement())
+    title    String
+    author   User?  @relation(fields: [authorId], references: [id])
+    authorId Int?
+
+    @@validate(authorId != null, 'needs an author')
+}
+`);
+        const user = await db.user.create({ data: {} });
+        const post = await db.post.create({ data: { title: 'a', authorId: user.id } });
+
+        await expect(db.post.update({ where: { id: post.id }, data: { title: 'b' } })).resolves.toMatchObject({
+            title: 'b',
+        });
+        await expect(
+            db.post.update({ where: { id: post.id }, data: { author: { disconnect: true } } }),
+        ).toBeRejectedByValidation(['needs an author']);
+    });
+
+    it('accepts a nested update of a relation whose foreign key a rule references', async () => {
+        const db = await createTestClient(`
+model User {
+    id    Int     @id @default(autoincrement())
+    name  String?
+    posts Post[]
+}
+
+model Post {
+    id       Int    @id @default(autoincrement())
+    title    String
+    author   User?  @relation(fields: [authorId], references: [id])
+    authorId Int?
+
+    @@validate(authorId != null, 'needs an author')
+}
+`);
+        const user = await db.user.create({ data: {} });
+        const post = await db.post.create({ data: { title: 'a', authorId: user.id } });
+
+        await expect(
+            db.post.update({
+                where: { id: post.id },
+                data: { author: { update: { name: 'x' } } },
+                include: { author: true },
+            }),
+        ).resolves.toMatchObject({ authorId: user.id, author: { name: 'x' } });
+        await expect(
+            db.post.update({
+                where: { id: post.id },
+                data: { author: { update: { name: 'y' }, disconnect: true } },
+            }),
+        ).toBeRejectedByValidation(['needs an author']);
+    });
+
+    it('checks a rule that references a field through this', async () => {
+        const db = await createTestClient(`
+model Item {
+    id     Int    @id @default(autoincrement())
+    name   String
+    amount Int
+
+    @@validate(this.amount > 0, 'amount must be positive')
+}
+`);
+        const item = await db.item.create({ data: { name: 'a', amount: 1 } });
+
+        await expect(db.item.update({ where: { id: item.id }, data: { name: 'b' } })).resolves.toMatchObject({
+            name: 'b',
+        });
+        await expect(db.item.update({ where: { id: item.id }, data: { amount: -1 } })).toBeRejectedByValidation([
+            'amount must be positive',
+        ]);
+    });
+});