Realm Migration Example

BaseMigration.java

package com.example.sample.orm.migration;

import com.google.common.base.Preconditions;
import com.example.sample.framework.base.ui.AppLogger;

import java.lang.reflect.Field;
import java.util.HashMap;
import java.util.Map;

import io.realm.Realm;
import io.realm.RealmMigration;
import io.realm.RealmObject;
import io.realm.internal.ColumnType;
import io.realm.internal.Table;

/**
 * User: mohit
 * Date: 13/4/15
 */
public abstract class BaseMigration implements RealmMigration {
    protected Map<String, Long> columnMap = new HashMap<>();

    protected Field[] fields;

    protected Class<? extends RealmObject> klass;

    protected Table migrationTable;

    /**
     * Need to call this method before anything else
     */
    void prepareMigration(Realm realm, Class<? extends RealmObject> klass) {
        this.migrationTable = realm.getTable(klass);
        this.klass = klass;
        this.fields = this.klass.getDeclaredFields();
    }

    void addColumns() {
        Preconditions.checkNotNull(migrationTable);
        Preconditions.checkNotNull(fields);
        Preconditions.checkNotNull(klass);
        for (Field field : fields) {
            String fieldName = field.getName();
            String type = field.getType().getSimpleName().toLowerCase();
            ColumnType columnType = ColumnType.INTEGER;
            switch (type) {
                // Add more types here
                case "long":
                    columnType = ColumnType.INTEGER;
                    break;
                case "string":
                    columnType = ColumnType.STRING;
                    break;
            }
            long index = MigrationUtils.getIndexForProperty(migrationTable, fieldName);
            if (index == -1) {
                // Does not exist, add it
                index = migrationTable.addColumn(columnType, fieldName);
            }
            columnMap.put(fieldName, index);
        }
    }

    void removeColumns() {
        long columnCount = migrationTable.getColumnCount();
        HashMap<String, Integer> deleteableCols = new HashMap<>();
        for (int i = 0; i < columnCount; i++) {
            String columnName = migrationTable.getColumnName(i);
            if (!columnMap.containsKey(columnName)) {
                deleteableCols.put(columnName, i);
            }
        }

        for (String column : deleteableCols.keySet()) {
            long index = MigrationUtils.getIndexForProperty(migrationTable, column);
            migrationTable.removeColumn(index);
        }
    }

    void setString(String fieldName, String value, long index) {
        migrationTable.setString(migrationTable.getColumnIndex(fieldName), index, value);
    }

    void setLong(String fieldName, Long value, long index) {
        migrationTable.setLong(migrationTable.getColumnIndex(fieldName), index, value);
    }

    long getLong(String fieldName, long index) {
        return migrationTable.getLong(migrationTable.getColumnIndex(fieldName), index);
    }
}

SampleMigration.java

package com.example.sample.orm.migration;

import android.text.TextUtils;

import com.google.common.base.Preconditions;

import org.json.JSONException;
import org.json.JSONObject;

import java.util.HashSet;

import io.realm.Realm;

/**
 * User: mohit
 * Date: 13/4/15
 */
public class SampleDataMigration extends BaseMigration {

    @Override
    public long execute(Realm realm, long version) {
      /**
       * Sample of how to use the migration, you don't need to care about removal/addition of columns, that is handled.
       */ 
        Preconditions.checkArgument(version == 0);
        Preconditions.checkNotNull(realm);
        prepareMigration(realm, Attachment.class);
        removeNonUniqueData(realm);
        addColumns();
        migrationTable.setPrimaryKey("id");
        removeColumns();
        realm.refresh();
        // Within a Realm Migration you cannot do a RealmQuery, because the migration
        // is incomplete, therefore we need to access data using this way
        // My model previously was storing non-structured data, JSON as string
        // I changed it to same structure as the json
        AppLogger.i("Migrating Existing data");
        for (int i = 0; i < migrationTable.size(); i++) {
            try {
                JSONObject contentObj = new JSONObject(migrationTable.getString(columnMap.get("content"), i));
                String type = contentObj.getString("tag")
                contentObj.put("resolvedType", type);
                if (contentObj.has("created_at")) {
                    contentObj.put("createTimestamp", DateUtils.getMiliEpochFromDate(contentObj.getString("created_at")));
                } else {
                    contentObj.put("createTimeStamp", getLong("createTimestamp", i));
                }
                // Migrate each object
                populateUsingJsonObject(contentObj, i);
            } catch (Exception e) {
                AppLogger.e(e);
            }
        }
        version++;
        return version;
    }

    /**
     * This is for the preparation of primary key index
     */
    private void removeNonUniqueData(Realm realm) {
      // Remove duplicate data you think should not exist
    }


    void populateUsingJsonObject(JSONObject json, long index)
            throws JSONException {
        // Migrate Data here
    }
}