Support nullable timestamps on MySQL (#3100)

* Add test to recreate the issue

* Improve test, add some documentation

* Improve test

* Add explicit nullable flag to column

* Set nullability explicitly on MySQL to deal with timestamp edge case

* Use same assertion for everything

src/dialects/mysql/schema/columncompiler.js

@@ -117,6 +117,13 @@ Object.assign(ColumnCompiler_MySQL.prototype, {
   // ------
 
   defaultTo(value) {
+    // MySQL defaults to null by default, but breaks down if you pass it explicitly
+    // Note that in MySQL versions up to 5.7, logic related to updating
+    // timestamps when no explicit value is passed is quite insane - https://dev.mysql.com/doc/refman/5.7/en/server-system-variables.html#sysvar_explicit_defaults_for_timestamp
+    if (value === null || value === undefined) {
+      return;
+    }
+
     const defaultVal = ColumnCompiler_MySQL.super_.prototype.defaultTo.apply(
       this,
       arguments

src/dialects/mysql/schema/tablecompiler.js

@@ -97,6 +97,9 @@ assign(TableCompiler_MySQL.prototype, {
 
                 if (String(column.Null).toUpperCase() !== 'YES') {
                   sql += ` NOT NULL`;
+                } else {
+                  // This doesn't matter for most cases except Timestamp, where this is important
+                  sql += ` NULL`;
                 }
                 if (column.Default !== void 0 && column.Default !== null) {
                   sql += ` DEFAULT '${column.Default}'`;

test/integration/migrate/index.js

@@ -19,6 +19,30 @@ module.exports = function(knex) {
   });
 
   describe('knex.migrate', function() {
+    it('should not fail on null default for timestamp', () => {
+      return knex.migrate
+        .latest({
+          directory: 'test/integration/migrate/null_timestamp_default',
+        })
+        .then(() => {
+          return knex.into('null_date').insert({});
+        })
+        .then(() => {
+          return knex
+            .from('null_date')
+            .select()
+            .first();
+        })
+        .then((rows) => {
+          expect(rows.deleted_at).to.equal(null);
+        })
+        .then(() => {
+          return knex.migrate.rollback({
+            directory: 'test/integration/migrate/null_timestamp_default',
+          });
+        });
+    });
+
     it('should not fail drop-and-recreate-column operation when using promise chain', () => {
       return knex.migrate
         .latest({

test/integration/migrate/null_timestamp_default/01_create.js

@@ -0,0 +1,14 @@
+exports.up = function(knex, Promise) {
+  return Promise.all([
+    knex.schema.createTable('null_date', function(t) {
+      t.increments('id').primary();
+      t.timestamp('deleted_at')
+        .nullable()
+        .defaultTo(null);
+    }),
+  ]);
+};
+
+exports.down = (knex) => {
+  return knex.schema.dropTable('null_date');
+};