feat(schema): add schema-level readConcern option to apply default readConcern for all queries

docs/guide.md

@@ -559,6 +559,7 @@ Valid options:
 * [methods](#methods)
 * [query](#query-helpers)
 * [autoSearchIndex](#autoSearchIndex)
+* [readConcern](#readConcern)
 
 <h2 id="autoIndex"><a href="#autoIndex">option: autoIndex</a></h2>
 
@@ -1473,6 +1474,24 @@ schema.searchIndex({
 const Test = mongoose.model('Test', schema);
 ```
 
+<h2 id="readConcern">
+  <a href="#readConcern">
+    option: readConcern
+  </a>
+</h2>
+
+[Read concerns](https://www.mongodb.com/docs/manual/reference/read-concern/) are similar to [`writeConcern`](#writeConcern), but for read operations like `find()` and `findOne()`.
+To set a default `readConcern`, pass the `readConcern` option to the schema constructor as follows.
+
+```javascript
+const eventSchema = new mongoose.Schema(
+  { name: String },
+  {
+    readConcern: { level: 'available' } // <-- set default readConcern for all queries
+  }
+);
+```
+
 <h2 id="es6-classes"><a href="#es6-classes">With ES6 Classes</a></h2>
 
 Schemas have a [`loadClass()` method](api/schema.html#schema_Schema-loadClass)

lib/helpers/schema/applyReadConcern.js

@@ -0,0 +1,22 @@
+'use strict';
+
+const get = require('../get');
+
+module.exports = function applyReadConcern(schema, options) {
+  if (options.readConcern !== undefined) {
+    return;
+  }
+
+  // Don't apply default read concern to operations in transactions,
+  // because you shouldn't set read concern on individual operations
+  // within a transaction.
+  // See: https://www.mongodb.com/docs/manual/reference/read-concern/
+  if (options && options.session && options.session.transaction) {
+    return;
+  }
+
+  const level = get(schema, 'options.readConcern.level', null);
+  if (level != null) {
+    options.readConcern = { level };
+  }
+};

lib/model.js

@@ -27,6 +27,7 @@ const applyEmbeddedDiscriminators = require('./helpers/discriminator/applyEmbedd
 const applyHooks = require('./helpers/model/applyHooks');
 const applyMethods = require('./helpers/model/applyMethods');
 const applyProjection = require('./helpers/projection/applyProjection');
+const applyReadConcern = require('./helpers/schema/applyReadConcern');
 const applySchemaCollation = require('./helpers/indexes/applySchemaCollation');
 const applyStaticHooks = require('./helpers/model/applyStaticHooks');
 const applyStatics = require('./helpers/model/applyStatics');
@@ -417,6 +418,8 @@ Model.prototype.$__handleSave = function(options, callback) {
         where[key] = val;
       }
     }
+
+    applyReadConcern(this.$__schema, optionsWithCustomValues);
     this.constructor.collection.findOne(where, optionsWithCustomValues)
       .then(documentExists => {
         const matchedCount = !documentExists ? 0 : 1;

lib/query.js

@@ -13,6 +13,7 @@ const QueryCursor = require('./cursor/queryCursor');
 const ValidationError = require('./error/validation');
 const { applyGlobalMaxTimeMS, applyGlobalDiskUse } = require('./helpers/query/applyGlobalOption');
 const handleReadPreferenceAliases = require('./helpers/query/handleReadPreferenceAliases');
+const applyReadConcern = require('./helpers/schema/applyReadConcern');
 const applyWriteConcern = require('./helpers/schema/applyWriteConcern');
 const cast = require('./cast');
 const castArrayFilters = require('./helpers/update/castArrayFilters');
@@ -1944,6 +1945,7 @@ Query.prototype._optionsForExec = function(model) {
   if (!model) {
     return options;
   }
+  applyReadConcern(model.schema, options);
   // Apply schema-level `writeConcern` option
   applyWriteConcern(model.schema, options);
 

lib/schema.js

@@ -66,6 +66,7 @@ const numberRE = /^\d+$/;
  * - [_id](https://mongoosejs.com/docs/guide.html#_id): bool - defaults to true
  * - [minimize](https://mongoosejs.com/docs/guide.html#minimize): bool - controls [document#toObject](https://mongoosejs.com/docs/api/document.html#Document.prototype.toObject()) behavior when called manually - defaults to true
  * - [read](https://mongoosejs.com/docs/guide.html#read): string
+ * - [readConcern](https://mongoosejs.com/docs/guide.html#readConcern): object - defaults to null, use to set a default [read concern](https://www.mongodb.com/docs/manual/reference/read-concern/) for all queries.
  * - [writeConcern](https://mongoosejs.com/docs/guide.html#writeConcern): object - defaults to null, use to override [the MongoDB server's default write concern settings](https://www.mongodb.com/docs/manual/reference/write-concern/)
  * - [shardKey](https://mongoosejs.com/docs/guide.html#shardKey): object - defaults to `null`
  * - [strict](https://mongoosejs.com/docs/guide.html#strict): bool - defaults to true

test/schema.test.js

@@ -3237,4 +3237,25 @@ describe('schema', function() {
     assert.equal(doc.element, '#hero');
     assert.ok(doc instanceof ClickedModel);
   });
+
+  it('supports schema-level readConcern (gh-14511)', async function() {
+    const eventSchema = new mongoose.Schema({
+      name: String
+    }, { readConcern: { level: 'available' } });
+    const Event = db.model('Test', eventSchema);
+
+    let q = Event.find();
+    let options = q._optionsForExec();
+    assert.deepStrictEqual(options.readConcern, { level: 'available' });
+
+    q = Event.find().setOptions({ readConcern: { level: 'local' } });
+    options = q._optionsForExec();
+    assert.deepStrictEqual(options.readConcern, { level: 'local' });
+
+    q = Event.find().setOptions({ readConcern: null });
+    options = q._optionsForExec();
+    assert.deepStrictEqual(options.readConcern, null);
+
+    await q;
+  });
 });

types/schemaoptions.d.ts

@@ -120,6 +120,10 @@ declare module 'mongoose' {
      * to all queries derived from a model.
      */
     read?: string;
+    /**
+     * Set a default readConcern for all queries at the schema level
+     */
+    readConcern?: { level: 'local' | 'available' | 'majority' | 'snapshot' | 'linearizable' }
     /** Allows setting write concern at the schema level. */
     writeConcern?: WriteConcern;
     /** defaults to true. */