Implemented distinct/distinctOn

lib/database.js

@@ -56,6 +56,17 @@ const buildWhere = (conditions, firstArgIndex = 1) => {
   return { clause: disjunction.join(' OR '), args };
 };
 
+const buildSelect = ({ distinct = false, distinctOn = [] }) => {
+  let select = 'SELECT';
+  if (distinctOn.length > 0) {
+    const fields = distinctOn.map((field) => `"${field}"`).join(', ');
+    select = `SELECT DISTINCT ON (${fields})`;
+  } else if (distinct) {
+    select = 'SELECT DISTINCT';
+  }
+  return select;
+};
+
 const updates = (delta, firstArgIndex = 1) => {
   const clause = [];
   const args = [];
@@ -103,6 +114,19 @@ class Query {
     return this;
   }
 
+  distinct(distinct = true) {
+    if (this.options.distinctOn)
+      Reflect.deleteProperty(this.options, 'distinctOn');
+    this.options.distinct = distinct;
+    return this;
+  }
+
+  distinctOn(fields = []) {
+    if (this.options.distinct) Reflect.deleteProperty(this.options, 'distinct');
+    this.options.distinctOn = fields;
+    return this;
+  }
+
   limit(count) {
     this.options.limit = count;
     return this;
@@ -116,14 +140,15 @@ class Query {
   prepare() {
     const args = [];
     const { table, fields, where, options } = this;
+    const { order, desc, limit, offset, distinct, distinctOn } = options;
+    const select = buildSelect({ distinct, distinctOn });
     const names = buildFields(fields);
-    const sql = [`SELECT ${names} FROM "${table}"`];
+    const sql = [`${select} ${names} FROM "${table}"`];
     if (where.length !== 0) {
       const cond = buildWhere(where);
       sql.push('WHERE ' + cond.clause);
       args.push(...cond.args);
     }
-    const { order, desc, limit, offset } = options;
     if (order) sql.push('ORDER BY "' + order.join('", "') + '"');
     if (desc) sql.push('ORDER BY "' + desc.join('", "') + '" DESC');
     if (limit) sql.push('LIMIT ' + limit);

metasql.d.ts

@@ -59,6 +59,8 @@ export class Query {
   );
   order(field: string | Array<string>): Query;
   desc(field: string | Array<string>): Query;
+  distinct(distinct: boolean): Query;
+  distinctOn(fields: Array<string>): Query;
   limit(count: number): Query;
   offset(count: number): Query;
   then(

test/sql.js

@@ -349,4 +349,14 @@ const metadomain = require('metadomain');
     await db.delete('Counter', { counterId });
     test.end();
   });
+
+  metatests.test('query.distinct/distinctOn', async (test) => {
+    const res1 = await db.select('City', ['countryId']).distinct();
+    test.strictSame(res1.length, 9);
+
+    const res2 = await db.select('City').distinctOn(['countryId']);
+    test.strictSame(res2.length, 9);
+
+    test.end();
+  });
 })();