diff --git a/addons/api/addon/generated/models/host-set.js b/addons/api/addon/generated/models/host-set.js
index 5793cc4a5..6ce3eb882 100644
--- a/addons/api/addon/generated/models/host-set.js
+++ b/addons/api/addon/generated/models/host-set.js
@@ -9,7 +9,8 @@ export default class GeneratedHostSetModel extends BaseModel {
   // =attributes
 
   @attr('string', {
-    description: 'The type of the resource, to help differentiate schemas'
+    description: 'The type of the resource, to help differentiate schemas',
+    readOnly: true
   }) type;
 
   @attr('string', {
@@ -36,7 +37,8 @@ export default class GeneratedHostSetModel extends BaseModel {
   version;
 
   @attr('number', {
-    description: 'The total count of hosts in this host set\nOutput only.'
+    description: 'The total count of hosts in this host set\nOutput only.',
+    readOnly: true
   }) size;
 
 }
diff --git a/addons/api/addon/models/host-set.js b/addons/api/addon/models/host-set.js
index 01a376f6b..bdfb07214 100644
--- a/addons/api/addon/models/host-set.js
+++ b/addons/api/addon/models/host-set.js
@@ -1,3 +1,30 @@
 import GeneratedHostSetModel from '../generated/models/host-set';
+import { fragmentArray } from 'ember-data-model-fragments/attributes';
 
-export default class HostSetModel extends GeneratedHostSetModel {}
+export default class HostSetModel extends GeneratedHostSetModel {
+
+  // =attributes
+
+  /**
+   * Grants is read-only _most_ under normal circumstances.  But grants can
+   * be persisted via a dedicated call to `saveGrants()`.
+   */
+  @fragmentArray('fragment-string', {readOnly: true}) host_ids;
+
+  // =methods
+
+  /**
+   * Saves the `host_ids` array on the host set via the `set-hosts` method.
+   * See serializer and adapter for more information.
+   * @return {Promise}
+   */
+  saveHostIDs() {
+    return this.save({
+      adapterOptions: {
+        method: 'set-hosts',
+        serializeHostIDs: true
+      }
+    });
+  }
+
+}
diff --git a/addons/api/addon/serializers/host-set.js b/addons/api/addon/serializers/host-set.js
new file mode 100644
index 000000000..24d2fff60
--- /dev/null
+++ b/addons/api/addon/serializers/host-set.js
@@ -0,0 +1,52 @@
+import ApplicationSerializer from './application';
+
+export default class HostSetSerializer extends ApplicationSerializer {
+
+  // =methods
+
+  /**
+   * If `adapterOptions.serializeHostIDs` is true, the serialization should
+   * include **only host_ids** and the version.  Normally, these are not
+   * serialized.
+   * @override
+   * @param {Snapshot} snapshot
+   * @return {object}
+   */
+  serialize(snapshot) {
+    const serializeHostIDs = snapshot?.adapterOptions?.serializeHostIDs;
+    let serialized = super.serialize(...arguments);
+    if (serializeHostIDs) serialized = this.serializeWithHostIDs(snapshot);
+    return serialized;
+  }
+
+  /**
+   * Returns a payload containing only the host_ids array.
+   * @param {Snapshot} snapshot
+   * @return {object}
+   */
+  serializeWithHostIDs(snapshot) {
+    return {
+      version: snapshot.attr('version'),
+      host_ids: snapshot.attr('host_ids').map(hostID => hostID.attr('value'))
+    };
+  }
+
+  /**
+   * Converts string arrays to object arrays for use with `fragment-string`.
+   * E.g. `"foo"` -> `{value: "foo"}`
+   * @override
+   * @param {Model} typeClass
+   * @param {Object} hash
+   * @return {Object}
+   */
+  normalize(modelClass, resourceHash) {
+    // TODO:  can fragment string normalization be handled generically?
+    // TODO: host_ids might actually be attributes.host_ids
+    if (resourceHash.host_ids) {
+      resourceHash.host_ids =
+        resourceHash.host_ids.map(value => ({ value }));
+    }
+    return super.normalize(modelClass, resourceHash);
+  }
+
+}
diff --git a/addons/api/app/serializers/host-set.js b/addons/api/app/serializers/host-set.js
new file mode 100644
index 000000000..8514878c2
--- /dev/null
+++ b/addons/api/app/serializers/host-set.js
@@ -0,0 +1 @@
+export { default } from 'api/serializers/host-set';
diff --git a/addons/api/tests/unit/serializers/host-set-test.js b/addons/api/tests/unit/serializers/host-set-test.js
new file mode 100644
index 000000000..13b6caa0b
--- /dev/null
+++ b/addons/api/tests/unit/serializers/host-set-test.js
@@ -0,0 +1,76 @@
+import { module, test } from 'qunit';
+import { setupTest } from 'ember-qunit';
+
+module('Unit | Serializer | host set', function(hooks) {
+  setupTest(hooks);
+
+  test('it serializes host sets normally, without host_ids', function (assert) {
+    assert.expect(1);
+    const store = this.owner.lookup('service:store');
+    const serializer = store.serializerFor('host-set');
+    const record = store.createRecord('host-set', {
+      name: 'Host Set 1',
+      description: 'Description',
+      host_ids: [{value: '1'}, {value: '2'}, {value: '3'}],
+      version: 1
+    });
+    const snapshot = record._createSnapshot();
+    snapshot.adapterOptions = {};
+    const serializedRecord = serializer.serialize(snapshot);
+    assert.deepEqual(serializedRecord, {
+      name: 'Host Set 1',
+      description: 'Description',
+      version: 1
+    });
+  });
+
+  test('it serializes only host_ids when `adapterOptions.serializeHostIDs` is true', function (assert) {
+    assert.expect(1);
+    const store = this.owner.lookup('service:store');
+    const serializer = store.serializerFor('host-set');
+    const record = store.createRecord('host-set', {
+      name: 'Host Set 1',
+      description: 'Description',
+      host_ids: [{value: '1'}, {value: '2'}, {value: '3'}],
+      version: 1
+    });
+    const snapshot = record._createSnapshot();
+    snapshot.adapterOptions = {
+      serializeHostIDs: true
+    };
+    const serializedRecord = serializer.serialize(snapshot);
+    assert.deepEqual(serializedRecord, {
+      host_ids: ['1', '2', '3'],
+      version: 1
+    });
+  });
+
+  test('it normalizes records with array fields', function (assert) {
+    assert.expect(1);
+    const store = this.owner.lookup('service:store');
+    const serializer = store.serializerFor('host-set');
+    const hostSetModelClass = store.createRecord('host-set').constructor;
+    const payload = {
+      id: '1',
+      name: 'Host Set 1',
+      host_ids: ['1', '2', '3']
+    };
+    const normalized = serializer.normalizeSingleResponse(
+      store,
+      hostSetModelClass,
+      payload
+    );
+    assert.deepEqual(normalized, {
+      included: [],
+      data: {
+        id: '1',
+        type: 'host-set',
+        attributes: {
+          name: 'Host Set 1',
+          host_ids: [{value: '1'}, {value: '2'}, {value: '3'}],
+        },
+        relationships: {},
+      },
+    });
+  });
+});
diff --git a/ui/core/mirage/factories/host-catalog.js b/ui/core/mirage/factories/host-catalog.js
index 02feb756d..b6ce98947 100644
--- a/ui/core/mirage/factories/host-catalog.js
+++ b/ui/core/mirage/factories/host-catalog.js
@@ -6,10 +6,8 @@ export default factory.extend({
   withChildren: trait({
     afterCreate(hostCatalog, server) {
       const scope = hostCatalog.scope;
-      // Hosts and host sets are supposed to be associated to a host catalog.
-      // For simplicity of mocks, we don't actually associate them here.
-      server.createList('host-set', 3, { scope, hostCatalog });
-      server.createList('host', 10, { scope, hostCatalog });
+      const hosts = server.createList('host', 10, { scope, hostCatalog });
+      server.createList('host-set', 3, { scope, hostCatalog, hosts });
     }
   })
 
diff --git a/ui/core/mirage/models/host-set.js b/ui/core/mirage/models/host-set.js
index 89aba81e6..2f78f5510 100644
--- a/ui/core/mirage/models/host-set.js
+++ b/ui/core/mirage/models/host-set.js
@@ -1,7 +1,8 @@
-import { Model, belongsTo } from 'ember-cli-mirage';
+import { Model, belongsTo, hasMany } from 'ember-cli-mirage';
 
 export default Model.extend({
 
-  hostCatalog: belongsTo()
+  hostCatalog: belongsTo(),
+  hosts: hasMany()
 
 });
diff --git a/ui/core/mirage/serializers/host-set.js b/ui/core/mirage/serializers/host-set.js
new file mode 100644
index 000000000..9c0fff2c8
--- /dev/null
+++ b/ui/core/mirage/serializers/host-set.js
@@ -0,0 +1,18 @@
+import ApplicationSerializer from './application';
+import { underscore } from '@ember/string';
+
+export default ApplicationSerializer.extend({
+  modelName: 'host-set',
+
+  keyForRelationshipIds(relationshipName) {
+    return `${this._container.inflector.singularize(
+      underscore(relationshipName)
+    )}_ids`;
+  },
+
+  _hashForModel(model) {
+    const json = ApplicationSerializer.prototype._hashForModel.apply(this, arguments);
+    json.host_ids = model.hostIds;
+    return json;
+  }
+});
diff --git a/ui/core/mirage/serializers/host.js b/ui/core/mirage/serializers/host.js
new file mode 100644
index 000000000..ccd3994ca
--- /dev/null
+++ b/ui/core/mirage/serializers/host.js
@@ -0,0 +1,5 @@
+import ApplicationSerializer from './application';
+
+export default ApplicationSerializer.extend({
+  modelName: 'host'
+});