cayenne-server/src/main/java/org/apache/cayenne/query/QueryChain.java

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 *  to you under the Apache License, Version 2.0 (the
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package org.apache.cayenne.query;

import org.apache.cayenne.CayenneRuntimeException;
import org.apache.cayenne.map.DataMap;
import org.apache.cayenne.map.EntityResolver;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;

/**
 * A Query decorator for a collection of other queries. Note that QueryChain will always
 * return DataRows (that is if it returns data), as it has no way of knowing how to
 * convert the results to objects.
 * 
 * @since 1.2
 */
public class QueryChain implements Query {

    protected Collection<Query> chain;

    /**
     * Creates an empty QueryChain.
     */
    public QueryChain() {
    }

    /**
     * Creates a new QueryChain out of an array of queries.
     */
    public QueryChain(Query[] queries) {
        if (queries != null && queries.length > 0) {
            this.chain = new ArrayList<>(Arrays.asList(queries));
        }
    }

    /**
     * Creates a new QueryChain with a collection of Queries.
     */
    public QueryChain(Collection<? extends Query> queries) {
        if (queries != null && !queries.isEmpty()) {
            this.chain = new ArrayList<>(queries);
        }
    }

    /**
     * Adds a query to the chain.
     */
    public void addQuery(Query query) {
        if (chain == null) {
            chain = new ArrayList<>();
        }

        chain.add(query);
    }

    /**
     * Removes a query from the chain, returning true if the query was indeed present in
     * the chain and was removed.
     */
    public boolean removeQuery(Query query) {
        return (chain != null) && chain.remove(query);
    }

    public boolean isEmpty() {
        return chain == null || chain.isEmpty();
    }

    /**
     * Delegates routing to each individual query in the chain. If there is no queries,
     * this method does nothing.
     */
    public void route(QueryRouter router, EntityResolver resolver, Query substitutedQuery) {
        if (chain != null && !chain.isEmpty()) {
            for (Query q : chain) {
                q.route(router, resolver, substitutedQuery);
            }
        }
    }

    /**
     * Throws an exception as execution should've been delegated to the queries contained
     * in the chain.
     */
    public SQLAction createSQLAction(SQLActionVisitor visitor) {
        throw new CayenneRuntimeException("Chain doesn't support its own execution "
                + "and should've been split into separate queries during routing phase.");
    }

    /**
     * Returns default metadata.
     */
    public QueryMetadata getMetaData(EntityResolver resolver) {
        QueryMetadataWrapper wrapper = new QueryMetadataWrapper(DefaultQueryMetadata.defaultMetadata);
        wrapper.override(QueryMetadata.FETCHING_DATA_ROWS_PROPERTY, Boolean.TRUE);
        return wrapper;
    }
}