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MongoDB adapter for Calcite
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/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to you under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.calcite.adapter.mongodb;

import org.apache.calcite.adapter.java.JavaTypeFactory;
import org.apache.calcite.plan.RelOptCluster;
import org.apache.calcite.plan.RelOptCost;
import org.apache.calcite.plan.RelOptPlanner;
import org.apache.calcite.plan.RelTraitSet;
import org.apache.calcite.rel.RelNode;
import org.apache.calcite.rel.core.Project;
import org.apache.calcite.rel.metadata.RelMetadataQuery;
import org.apache.calcite.rel.type.RelDataType;
import org.apache.calcite.rex.RexNode;
import org.apache.calcite.util.Pair;
import org.apache.calcite.util.Util;

import com.google.common.collect.ImmutableList;

import org.checkerframework.checker.nullness.qual.Nullable;

import java.util.ArrayList;
import java.util.List;

/**
 * Implementation of {@link org.apache.calcite.rel.core.Project}
 * relational expression in MongoDB.
 */
public class MongoProject extends Project implements MongoRel {
  public MongoProject(RelOptCluster cluster, RelTraitSet traitSet,
      RelNode input, List projects, RelDataType rowType) {
    super(cluster, traitSet, ImmutableList.of(), input, projects, rowType);
    assert getConvention() == MongoRel.CONVENTION;
    assert getConvention() == input.getConvention();
  }

  @Deprecated // to be removed before 2.0
  public MongoProject(RelOptCluster cluster, RelTraitSet traitSet,
      RelNode input, List projects, RelDataType rowType, int flags) {
    this(cluster, traitSet, input, projects, rowType);
    Util.discard(flags);
  }

  @Override public Project copy(RelTraitSet traitSet, RelNode input,
      List projects, RelDataType rowType) {
    return new MongoProject(getCluster(), traitSet, input, projects,
        rowType);
  }

  @Override public @Nullable RelOptCost computeSelfCost(RelOptPlanner planner,
      RelMetadataQuery mq) {
    return super.computeSelfCost(planner, mq).multiplyBy(0.1);
  }

  @Override public void implement(Implementor implementor) {
    implementor.visitChild(0, getInput());

    final MongoRules.RexToMongoTranslator translator =
        new MongoRules.RexToMongoTranslator(
            (JavaTypeFactory) getCluster().getTypeFactory(),
            MongoRules.mongoFieldNames(getInput().getRowType()));
    final List items = new ArrayList<>();
    for (Pair pair : getNamedProjects()) {
      final String name = pair.right;
      final String expr = pair.left.accept(translator);
      items.add(expr.equals("'$" + name + "'")
          ? MongoRules.maybeQuote(name) + ": 1"
          : MongoRules.maybeQuote(name) + ": " + expr);
    }
    final String findString = Util.toString(items, "{", ", ", "}");
    final String aggregateString = "{$project: " + findString + "}";
    final Pair op = Pair.of(findString, aggregateString);
    implementor.add(op.left, op.right);
  }
}