迁移时如何删除领域？

Question

这就是我想要的：

public class CustomRealmMigration implements RealmMigration {
    // Current version
    private static final long SCHEMA_VERSION = 4;

    private final Context mContext;

    public CustomRealmMigration(Context context) {
        mContext = context;
    }

    @Override
    public long execute(Realm realm, long version) {
        if (SCHEMA_VERSION < version) {
            // Rollback, not allow
            deleteRealm();
            return SCHEMA_VERSION;
        }
        return version;
    }

    private void deleteRealm() {
        RealmConfiguration realmConfiguration = new RealmConfiguration.Builder(mContext)
                .name(Realm.DEFAULT_REALM_NAME)
                .build();
        Realm.deleteRealm(realmConfiguration);
    }
}

有错误：

Caused by: java.lang.IllegalStateException: It's not allowed to delete the file associated with an open Realm. Remember to close() all the instances of the Realm before deleting its file.

迁移时如何删除 Realm？或者如何在其他地方获取旧版本的 Realm？

Answer 1

如果您不想在架构更改时迁移数据而只是重置/清除数据库，那么您可以执行以下操作：

RealmConfiguration realmConfiguration = new RealmConfiguration.Builder(mContext)
                .name(Realm.DEFAULT_REALM_NAME)
                .deleteRealmIfMigrationRequired()
                .build();

这也意味着您不必提供任何迁移代码。

Answer 2

public class Migration implements RealmMigration {
    @Override
    public long execute(Realm realm, long version) {

        /*
            // Version 0
            class Person
                String firstName;
                String lastName;
                int    age;

            // Version 1
            class Person
                String fullName;        // combine firstName and lastName into single field
                int age;
        */

        // Migrate from version 0 to version 1
        if (version == 0) {
            Table personTable = realm.getTable(Person.class);

            long fistNameIndex = getIndexForProperty(personTable, "firstName");
            long lastNameIndex = getIndexForProperty(personTable, "lastName");
            long fullNameIndex = personTable.addColumn(ColumnType.STRING, "fullName");
            for (int i = 0; i < personTable.size(); i++) {
                personTable.setString(fullNameIndex, i, personTable.getString(fistNameIndex, i) + " " +
                        personTable.getString(lastNameIndex, i));
            }
            personTable.removeColumn(getIndexForProperty(personTable, "firstName"));
            personTable.removeColumn(getIndexForProperty(personTable, "lastName"));
            version++;
        }

        /*
            // Version 2
                class Pet                   // add a new model class
                    String name;
                    String type;

                class Person
                    String fullName;
                    int age;
                    RealmList<Pet> pets;    // add an array property

        */
        // Migrate from version 1 to version 2
        if (version == 1) {
            Table personTable = realm.getTable(Person.class);
            Table petTable = realm.getTable(Pet.class);
            petTable.addColumn(ColumnType.STRING, "name");
            petTable.addColumn(ColumnType.STRING, "type");
            long petsIndex = personTable.addColumnLink(ColumnType.LINK_LIST, "pets", petTable);
            long fullNameIndex = getIndexForProperty(personTable, "fullName");

            for (int i = 0; i < personTable.size(); i++) {
                if (personTable.getString(fullNameIndex, i).equals("JP McDonald")) {
                    personTable.getUncheckedRow(i).getLinkList(petsIndex).add(petTable.add("Jimbo", "dog"));
                }
            }
            version++;
        }

        /*
            // Version 3
                class Pet
                    String name;
                    int type;               // type becomes int

                class Person
                    String fullName;
                    RealmList<Pet> pets;    // age and pets re-ordered
                    int age;
        */
        // Migrate from version 2 to version 3
        if (version == 2) {
            Table petTable = realm.getTable(Pet.class);
            long oldTypeIndex = getIndexForProperty(petTable, "type");
            long typeIndex = petTable.addColumn(ColumnType.INTEGER, "type");
            for (int i = 0; i < petTable.size(); i++) {
                String type = petTable.getString(oldTypeIndex, i);
                if (type.equals("dog")) {
                    petTable.setLong(typeIndex, i, 1);
                }
                else if (type.equals("cat")) {
                    petTable.setLong(typeIndex, i, 2);
                }
                else if (type.equals("hamster")) {
                    petTable.setLong(typeIndex, i, 3);
                }
            }
            petTable.removeColumn(oldTypeIndex);
            version++;
        }
        return version;
    }

    private long getIndexForProperty(Table table, String name) {
        for (int i = 0; i < table.getColumnCount(); i++) {
            if (table.getColumnName(i).equals(name)) {
                return i;
            }
        }
        return -1;
    }
}



And in Activity 

 copyBundledRealmFile(this.getResources().openRawResource(R.raw.default0), "default0");
        copyBundledRealmFile(this.getResources().openRawResource(R.raw.default1), "default1");
        copyBundledRealmFile(this.getResources().openRawResource(R.raw.default2), "default2");

        // When you create a RealmConfiguration you can specify the version of the schema.
        // If the schema does not have that version a RealmMigrationNeededException will be thrown.
        RealmConfiguration config0 = new RealmConfiguration.Builder(this)
                .name("default0")
                .schemaVersion(3)
                .build();

        // You can then manually call Realm.migrateRealm().
        Realm.migrateRealm(config0, new Migration());
        realm = Realm.getInstance(config0);
        showStatus("Default0");
        showStatus(realm);
        realm.close();

        // Or you can add the migration code to the configuration. This will run the migration code without throwing
        // a RealmMigrationNeededException.
        RealmConfiguration config1 = new RealmConfiguration.Builder(this)
                .name("default1")
                .schemaVersion(3)
                .migration(new Migration())
                .build();

        realm = Realm.getInstance(config1); // Automatically run migration if needed
        showStatus("Default1");
        showStatus(realm);
        realm.close();

        // or you can set .deleteRealmIfMigrationNeeded() if you don't want to bother with migrations.
        // WARNING: This will delete all data in the Realm though.
        RealmConfiguration config2 = new RealmConfiguration.Builder(this)
                .name("default2")
                .schemaVersion(3)
                .deleteRealmIfMigrationNeeded()
                .build();

        realm = Realm.getInstance(config2);
        showStatus("default2");
        showStatus(realm);
        realm.close();