Add a method for overriding modelId in item settings.

fabric-item-api-v1/src/main/java/net/fabricmc/fabric/api/item/v1/FabricItem.java

@@ -22,6 +22,7 @@
 import net.minecraft.item.ItemStack;
 import net.minecraft.registry.entry.RegistryEntry;
 import net.minecraft.util.Hand;
+import net.minecraft.util.Identifier;
 
 import net.fabricmc.fabric.impl.item.FabricItemInternals;
 
@@ -143,5 +144,15 @@ default Item.Settings customDamage(CustomDamageHandler handler) {
 			FabricItemInternals.computeExtraData((Item.Settings) this).customDamage(handler);
 			return (Item.Settings) this;
 		}
+
+		/**
+		 * Sets the model of the item to static Identifier.
+		 *
+		 * @param modelId the model id item should use
+		 * @return this builder
+		 */
+		default Item.Settings modelId(Identifier modelId) {
+			return (Item.Settings) this;
+		}
 	}
 }

fabric-item-api-v1/src/main/java/net/fabricmc/fabric/mixin/item/ItemSettingsMixin.java

@@ -17,11 +17,22 @@
 package net.fabricmc.fabric.mixin.item;
 
 import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.Shadow;
 
 import net.minecraft.item.Item;
+import net.minecraft.registry.RegistryKeyedValue;
+import net.minecraft.util.Identifier;
 
 import net.fabricmc.fabric.api.item.v1.FabricItem;
 
 @Mixin(Item.Settings.class)
 public class ItemSettingsMixin implements FabricItem.Settings {
+	@Shadow
+	private RegistryKeyedValue<Item, Identifier> modelId;
+
+	@Override
+	public Item.Settings modelId(Identifier modelId) {
+		this.modelId = RegistryKeyedValue.fixed(modelId);
+		return FabricItem.Settings.super.modelId(modelId);
+	}
 }

fabric-item-api-v1/src/testmod/java/net/fabricmc/fabric/test/item/CustomModelIdTest.java

@@ -0,0 +1,36 @@
+/*
+ * Copyright (c) 2016, 2017, 2018, 2019 FabricMC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package net.fabricmc.fabric.test.item;
+
+import net.minecraft.item.Item;
+import net.minecraft.registry.Registries;
+import net.minecraft.registry.Registry;
+import net.minecraft.registry.RegistryKey;
+import net.minecraft.registry.RegistryKeys;
+import net.minecraft.util.Identifier;
+
+import net.fabricmc.api.ModInitializer;
+
+public class CustomModelIdTest implements ModInitializer {
+	public static final RegistryKey<Item> NOT_A_DIAMOND_KEY = RegistryKey.of(RegistryKeys.ITEM, Identifier.of("fabric-item-api-v1-testmod", "not_a_diamond"));
+	public static final Item NOT_A_DIAMOND = new Item(new Item.Settings().registryKey(NOT_A_DIAMOND_KEY).modelId(Identifier.ofVanilla("diamond")));
+
+	@Override
+	public void onInitialize() {
+		Registry.register(Registries.ITEM, NOT_A_DIAMOND_KEY, NOT_A_DIAMOND);
+	}
+}

fabric-item-api-v1/src/testmod/resources/fabric.mod.json

@@ -12,6 +12,7 @@
     "main": [
       "net.fabricmc.fabric.test.item.CustomDamageTest",
       "net.fabricmc.fabric.test.item.DefaultItemComponentTest",
+      "net.fabricmc.fabric.test.item.CustomModelIdTest",
       "net.fabricmc.fabric.test.item.ItemUpdateAnimationTest",
       "net.fabricmc.fabric.test.item.ArmorKnockbackResistanceTest",
       "net.fabricmc.fabric.test.item.CustomEnchantmentEffectsTest"