从DateTimeField迁移到DateField

Question

我有一个模型，其中包含一个created_at字段，它是一个models.DateTimeField。我想将它迁移到models.DateField，因为时间已经不需要了。我更改模式，运行makemigrations，并将其应用到我的数据库中。我愚蠢地假设数据将被迁移。没有，因为所有的created_at字段现在都是None。

如何转换现有数据以使日期仍然可用？

下面是生成的迁移文件：

# -*- coding: utf-8 -*-
# Generated by Django 1.10.5 on 2017-03-01 15:07
from __future__ import unicode_literals

import datetime
from django.db import migrations, models


class Migration(migrations.Migration):

    dependencies = [
        ('aion', '0003_auto_20160125_0948'),
    ]

    operations = [
        migrations.AlterField(
            model_name='assembly',
            name='created_at',
            field=models.DateField(default=datetime.date.today, verbose_name='date created'),
        ),
    ]

该数据库是一个运行在SQLite上的测试数据库。真正的运行在MySQL…上。

$ python manage.py sqlmigrate aion 0004_auto_20170301_1507
BEGIN;
--
-- Alter field created_at on assembly
--
ALTER TABLE "aion_assembly" RENAME TO "aion_assembly__old";
CREATE TABLE "aion_assembly" ("created_at" date NOT NULL, "description" text NOT NULL, "serial_number" integer NOT NULL PRIMARY KEY AUTOINCREMENT, "status" varchar(32) NOT NULL, "manufacturer_serial_number" varchar(128) NULL, "supplier_reference" varchar(128) NULL, "sap" varchar(128) NULL, "location_number_field" varchar(128) NULL, "customer_equipment_reference" varchar(128) NULL, "configurator_id" integer NULL REFERENCES "aion_configurator" ("id"), "created_by_id" integer NOT NULL REFERENCES "auth_user" ("id"), "customer_id" integer NULL REFERENCES "organisations_customer" ("id"), "locality_id" integer NULL REFERENCES "aion_location" ("id"), "project_code_id" integer NULL REFERENCES "project_codes_projectcode" ("id"), "supplier_id" integer NULL REFERENCES "organisations_supplier" ("id"), "contained_in_id" integer NULL REFERENCES "aion_assembly" ("serial_number"));
INSERT INTO "aion_assembly" ("status", "customer_equipment_reference", "manufacturer_serial_number", "description", "supplier_reference", "created_at", "created_by_id", "project_code_id", "contained_in_id", "locality_id", "supplier_id", "location_number_field", "customer_id", "serial_number", "configurator_id", "sap") SELECT "status", "customer_equipment_reference", "manufacturer_serial_number", "description", "supplier_reference", "created_at", "created_by_id", "project_code_id", "contained_in_id", "locality_id", "supplier_id", "location_number_field", "customer_id", "serial_number", "configurator_id", "sap" FROM "aion_assembly__old";
DROP TABLE "aion_assembly__old";
CREATE INDEX "aion_assembly_cc6edc4a" ON "aion_assembly" ("configurator_id");
CREATE INDEX "aion_assembly_e93cb7eb" ON "aion_assembly" ("created_by_id");
CREATE INDEX "aion_assembly_cb24373b" ON "aion_assembly" ("customer_id");
CREATE INDEX "aion_assembly_7e3ea948" ON "aion_assembly" ("locality_id");
CREATE INDEX "aion_assembly_f56bb38b" ON "aion_assembly" ("project_code_id");
CREATE INDEX "aion_assembly_c5bcd634" ON "aion_assembly" ("supplier_id");
CREATE INDEX "aion_assembly_e71ddfe4" ON "aion_assembly" ("contained_in_id");
COMMIT;

Answer 1

SQLite是惊人的，但它的行为并不总是与其他数据库相同。有关SQLite的可能相关事实包括：

• SQLite列类型具有一定的动态性
• 不支持列修改

正如您所看到的，在使用SQLite时，Django实际上创建了一个新表，并在修改列时从旧表中复制数据。

我不太清楚SQLite在您的环境中有什么问题，但是由于您在生产中使用MySQL，所以我建议在开发中也使用MySQL。我们已经看到，这种迁移在MySQL下可以正常工作。