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'use strict';
module.exports = function prepareDiscriminatorPipeline(pipeline, schema, prefix) {  const discriminatorMapping = schema && schema.discriminatorMapping;  prefix = prefix || '';
  if (discriminatorMapping && !discriminatorMapping.isRoot) {    const originalPipeline = pipeline;    const filterKey = (prefix.length > 0 ? prefix + '.' : prefix) + discriminatorMapping.key;    const discriminatorValue = discriminatorMapping.value;
    // If the first pipeline stage is a match and it doesn't specify a `__t`    // key, add the discriminator key to it. This allows for potential    // aggregation query optimizations not to be disturbed by this feature.    if (originalPipeline[0] != null &&        originalPipeline[0].$match &&        (originalPipeline[0].$match[filterKey] === undefined || originalPipeline[0].$match[filterKey] === discriminatorValue)) {      originalPipeline[0].$match[filterKey] = discriminatorValue;      // `originalPipeline` is a ref, so there's no need for      // aggregate._pipeline = originalPipeline    } else if (originalPipeline[0] != null && originalPipeline[0].$geoNear) {      originalPipeline[0].$geoNear.query =          originalPipeline[0].$geoNear.query || {};      originalPipeline[0].$geoNear.query[filterKey] = discriminatorValue;    } else if (originalPipeline[0] != null && originalPipeline[0].$search) {      if (originalPipeline[1] && originalPipeline[1].$match != null) {        originalPipeline[1].$match[filterKey] = originalPipeline[1].$match[filterKey] || discriminatorValue;      } else {        const match = {};        match[filterKey] = discriminatorValue;        originalPipeline.splice(1, 0, { $match: match });      }    } else {      const match = {};      match[filterKey] = discriminatorValue;      originalPipeline.unshift({ $match: match });    }  }};