diff --git a/.travis.yml b/.travis.yml
index c6c68fa..2d360c0 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -19,9 +19,6 @@ matrix:
       before_script:
         - cd analytics
         - pip install -r requirements.txt
-        - sudo apt-get update
-        - sudo apt-get install -y 
-          apt-utils gnupg curl make g++ git
       script:
         - python -m unittest discover
 
diff --git a/server/package.json b/server/package.json
index b1ac1bf..51e37a5 100644
--- a/server/package.json
+++ b/server/package.json
@@ -21,7 +21,7 @@
   "homepage": "https://github.com/hastic/hastic-server#readme",
   "dependencies": {},
   "devDependencies": {
-    "@types/jest": "^23.1.1",
+    "@types/jest": "^23.3.14",
     "@types/koa": "^2.0.46",
     "@types/koa-bodyparser": "^4.2.0",
     "@types/koa-router": "^7.0.31",
diff --git a/server/spec/segments.jest.ts b/server/spec/segments.jest.ts
index 480be96..667208c 100644
--- a/server/spec/segments.jest.ts
+++ b/server/spec/segments.jest.ts
@@ -1,5 +1,5 @@
-import { TEST_ANALYTIC_UNIT_ID } from './utils_for_tests/analytic_units';
-import { buildSegments, clearSegmentsDB, convertSegmentsToTimeRanges } from './utils_for_tests/segments';
+import { TEST_ANALYTIC_UNIT_ID, createTestDB, clearTestDB } from './utils_for_tests/analytic_units';
+import { buildSegments, clearSegmentsDB } from './utils_for_tests/segments';
 
 import * as Segment from '../src/models/segment_model';
 
@@ -7,6 +7,11 @@ import * as _ from 'lodash';
 
 const INITIAL_SEGMENTS = buildSegments([[0, 1], [2, 3], [4, 5]]);
 
+beforeAll(async () => {
+  await clearTestDB();
+  await createTestDB();
+});
+
 beforeEach(async () => {
   await Segment.mergeAndInsertSegments(INITIAL_SEGMENTS);
 });
@@ -16,7 +21,7 @@ afterEach(async () => {
 });
 
 describe('mergeAndInsertSegments', function() {
-  it('Should be merged before insertion', async function() {
+  it('should be merged before insertion', async function() {
     const segmentsToInsert = buildSegments([[1, 2]]);
     await Segment.mergeAndInsertSegments(segmentsToInsert);
 
diff --git a/server/spec/utils_for_tests/analytic_units.ts b/server/spec/utils_for_tests/analytic_units.ts
index 45e4367..9c20c97 100644
--- a/server/spec/utils_for_tests/analytic_units.ts
+++ b/server/spec/utils_for_tests/analytic_units.ts
@@ -1,9 +1,10 @@
 import * as AnalyticUnit from '../../src/models/analytic_units';
+import * as AnalyticUnitCache from '../../src/models/analytic_unit_cache_model';
+
 
 import { Metric } from 'grafana-datasource-kit';
 
 import * as _ from 'lodash';
-import * as AnalyticUnitCache from '../../src/models/analytic_unit_cache_model';
 
 
 export const TEST_ANALYTIC_UNIT_ID: AnalyticUnit.AnalyticUnitId = 'testid';
diff --git a/server/spec/utils_for_tests/segments.ts b/server/spec/utils_for_tests/segments.ts
index 87bca00..9a5cdfb 100644
--- a/server/spec/utils_for_tests/segments.ts
+++ b/server/spec/utils_for_tests/segments.ts
@@ -16,5 +16,5 @@ export function convertSegmentsToTimeRanges(segments: Segment.Segment[]): number
 
 export async function clearSegmentsDB(): Promise<void> {
   const segments = await Segment.findMany(TEST_ANALYTIC_UNIT_ID, { labeled: false, deleted: false });
-  await Segment.removeSegments(segments.map(s => s.id));
+  await Segment.removeSegments(_.compact(segments.map(s => s.id)));
 }
diff --git a/server/src/models/analytic_unit_cache_model.ts b/server/src/models/analytic_unit_cache_model.ts
index 2f22c46..0be76cb 100644
--- a/server/src/models/analytic_unit_cache_model.ts
+++ b/server/src/models/analytic_unit_cache_model.ts
@@ -54,7 +54,7 @@ export class AnalyticUnitCache {
   }
 }
 
-export async function findById(id: AnalyticUnitId): Promise<AnalyticUnitCache> {
+export async function findById(id: AnalyticUnitId): Promise<AnalyticUnitCache | null> {
   let obj = await db.findOne(id);
   if(obj === null) {
     return null;
diff --git a/server/src/models/analytic_units/anomaly_analytic_unit_model.ts b/server/src/models/analytic_units/anomaly_analytic_unit_model.ts
index 4f4d3be..0802f5c 100644
--- a/server/src/models/analytic_units/anomaly_analytic_unit_model.ts
+++ b/server/src/models/analytic_units/anomaly_analytic_unit_model.ts
@@ -75,7 +75,7 @@ export class AnomalyAnalyticUnit extends AnalyticUnit {
 
   static fromObject(obj: any) {
     // TODO: remove duplication
-    let metric: Metric;
+    let metric: Metric | undefined = undefined;
     if (obj.metric !== undefined) {
       metric = Metric.fromObject(obj.metric);
     }
diff --git a/server/src/models/analytic_units/db.ts b/server/src/models/analytic_units/db.ts
index a0369a8..e5a8f6f 100644
--- a/server/src/models/analytic_units/db.ts
+++ b/server/src/models/analytic_units/db.ts
@@ -10,7 +10,7 @@ import * as _ from 'lodash';
 
 const db = makeDBQ(Collection.ANALYTIC_UNITS);
 
-export async function findById(id: AnalyticUnitId): Promise<AnalyticUnit> {
+export async function findById(id: AnalyticUnitId): Promise<AnalyticUnit | null> {
   let obj = await db.findOne(id);
   if (obj === null) {
     return null;
diff --git a/server/src/models/analytic_units/pattern_analytic_unit_model.ts b/server/src/models/analytic_units/pattern_analytic_unit_model.ts
index ea4ccf6..56f74ed 100644
--- a/server/src/models/analytic_units/pattern_analytic_unit_model.ts
+++ b/server/src/models/analytic_units/pattern_analytic_unit_model.ts
@@ -56,7 +56,7 @@ export class PatternAnalyticUnit extends AnalyticUnit {
 
   static fromObject(obj: any) {
     // TODO: remove duplication
-    let metric: Metric;
+    let metric: Metric | undefined = undefined;
     if(obj.metric !== undefined) {
       metric = Metric.fromObject(obj.metric);
     }
diff --git a/server/src/models/analytic_units/threshold_analytic_unit_model.ts b/server/src/models/analytic_units/threshold_analytic_unit_model.ts
index e241668..d5a830a 100644
--- a/server/src/models/analytic_units/threshold_analytic_unit_model.ts
+++ b/server/src/models/analytic_units/threshold_analytic_unit_model.ts
@@ -72,7 +72,7 @@ export class ThresholdAnalyticUnit extends AnalyticUnit {
 
   static fromObject(obj: any) {
     // TODO: remove duplication
-    let metric: Metric;
+    let metric: Metric | undefined = undefined;
     if (obj.metric !== undefined) {
       metric = Metric.fromObject(obj.metric);
     }
diff --git a/server/src/models/segment_model.ts b/server/src/models/segment_model.ts
index 3edc9a3..ee19920 100644
--- a/server/src/models/segment_model.ts
+++ b/server/src/models/segment_model.ts
@@ -114,8 +114,8 @@ export async function findMany(id: AnalyticUnitId, query: FindManyQuery): Promis
  * If `from` and `to` are undefined: @returns all segments
  */
 export async function findIntersectedSegments(
-  analyticUnitId: AnalyticUnit.AnalyticUnitId, 
-  from?: number, 
+  analyticUnitId: AnalyticUnit.AnalyticUnitId,
+  from?: number,
   to?: number
 ): Promise<Segment[]> {
   let query: FindManyQuery = {};
@@ -128,6 +128,9 @@ export async function findIntersectedSegments(
   return findMany(analyticUnitId, query);
 }
 
+
+// TODO: rewrite all this horrible function
+// TODO: use utils.segments.IntegerSegmentsSet
 /**
  * Merges an array of segments with ones existing in the DB
  * Inserts resulting segments into DB
@@ -142,6 +145,18 @@ export async function mergeAndInsertSegments(segments: Segment[]): Promise<{
     return { addedIds: [], removedIds: [] };
   }
   const analyticUnitId: AnalyticUnitId = segments[0].analyticUnitId;
+  const unit = await AnalyticUnit.findById(analyticUnitId);
+  if(unit === null) {
+    throw new Error('Can`t find analytic unit ' + analyticUnitId);
+  }
+  const cache = await AnalyticUnitCache.findById(analyticUnitId);
+
+  if(cache === null) {
+    throw new Error('Can`t find cache for analytic unit ' + analyticUnitId);
+  }
+
+  const detector = unit.detectorType;
+
   let segmentIdsToRemove: SegmentId[] = [];
   let segmentsToInsert: Segment[] = [];
 
@@ -156,11 +171,7 @@ export async function mergeAndInsertSegments(segments: Segment[]): Promise<{
       }
     }
 
-    let cache = await AnalyticUnitCache.findById(analyticUnitId);
-    let unit = await AnalyticUnit.findById(analyticUnitId);
-    const detector = unit.detectorType;
-
-    let intersectedSegments: Segment[];
+    let intersectedSegments: Segment[] = [];
     if(detector === AnalyticUnit.DetectorType.PATTERN) {
       intersectedSegments = await findMany(analyticUnitId, {
         to: { $gte: segment.from },
@@ -179,12 +190,24 @@ export async function mergeAndInsertSegments(segments: Segment[]): Promise<{
     }
 
     if(intersectedSegments.length > 0) {
-      let from = _.minBy(intersectedSegments.concat(segment), s => s.from).from;
-      let to = _.maxBy(intersectedSegments.concat(segment), s => s.to).to;
+      let intersectedIds = intersectedSegments.map(s => s.id);
+      let minFromSegment = _.minBy(intersectedSegments.concat(segment), s => s.from);
+      let maxToSegment = _.maxBy(intersectedSegments.concat(segment), s => s.to);
+
+      if(minFromSegment === undefined) {
+        throw new Error('minFromSegment is undefined');
+      }
+
+      if(maxToSegment === undefined) {
+        throw new Error('maxToSegment is undefined');
+      }
+
+      let from = minFromSegment.from;
+      let to = maxToSegment.to;
       let newSegment = Segment.fromObject(segment.toObject());
       newSegment.from = from;
       newSegment.to = to;
-      segmentIdsToRemove = segmentIdsToRemove.concat(intersectedSegments.map(s => s.id));
+      segmentIdsToRemove = segmentIdsToRemove.concat(_.compact(intersectedIds));
       segmentsToInsert.push(newSegment);
     } else {
       segmentsToInsert.push(segment);
diff --git a/server/src/services/data_service.ts b/server/src/services/data_service.ts
index 3f72970..20a4c0f 100644
--- a/server/src/services/data_service.ts
+++ b/server/src/services/data_service.ts
@@ -31,16 +31,24 @@ export type DBQ = {
   removeMany: (query: string[] | object) => Promise<number>
 }
 
+function nedbCollectionFromCollection(collection: Collection): nedb {
+  let nedbCollection = db.get(collection);
+  if(nedbCollection === undefined) {
+    throw new Error('Can`t find collection ' + collection);
+  }
+  return nedbCollection;
+}
+
 export function makeDBQ(collection: Collection): DBQ {
   return {
-    findOne: dbFindOne.bind(null, collection),
-    findMany: dbFindMany.bind(null, collection),
-    insertOne: dbInsertOne.bind(null, collection),
-    insertMany: dbInsertMany.bind(null, collection),
-    updateOne: dbUpdateOne.bind(null, collection),
-    updateMany: dbUpdateMany.bind(null, collection),
-    removeOne: dbRemoveOne.bind(null, collection),
-    removeMany: dbRemoveMany.bind(null, collection)
+    findOne: dbFindOne.bind(null, nedbCollectionFromCollection(collection)),
+    findMany: dbFindMany.bind(null, nedbCollectionFromCollection(collection)),
+    insertOne: dbInsertOne.bind(null, nedbCollectionFromCollection(collection)),
+    insertMany: dbInsertMany.bind(null, nedbCollectionFromCollection(collection)),
+    updateOne: dbUpdateOne.bind(null, nedbCollectionFromCollection(collection)),
+    updateMany: dbUpdateMany.bind(null, nedbCollectionFromCollection(collection)),
+    removeOne: dbRemoveOne.bind(null, nedbCollectionFromCollection(collection)),
+    removeMany: dbRemoveMany.bind(null, nedbCollectionFromCollection(collection))
   }
 }
 
@@ -58,6 +66,7 @@ function wrapIdsToQuery(query: string[] | object): any {
   return query;
 }
 
+// TODO: move to utils
 function isEmptyArray(obj: any): boolean {
   if(!Array.isArray(obj)) {
     return false;
@@ -67,24 +76,25 @@ function isEmptyArray(obj: any): boolean {
 
 const db = new Map<Collection, nedb>();
 
-let dbInsertOne = (collection: Collection, doc: object) => {
+
+async function dbInsertOne(nd: nedb, doc: object): Promise<string> {
   return new Promise<string>((resolve, reject) => {
-    db.get(collection).insert(doc, (err, newDoc: any) => {
+    nd.insert(doc, (err, newDoc: any) => {
       if(err) {
         reject(err);
       } else {
         resolve(newDoc._id);
       }
-    });
+    })
   });
 }
 
-let dbInsertMany = (collection: Collection, docs: object[]) => {
+async function dbInsertMany(nd: nedb, docs: object[]): Promise<string[]> {
   if(docs.length === 0) {
     return Promise.resolve([]);
   }
   return new Promise<string[]>((resolve, reject) => {
-    db.get(collection).insert(docs, (err, newDocs: any[]) => {
+    nd.insert(docs, (err, newDocs: any[]) => {
       if(err) {
         reject(err);
       } else {
@@ -94,12 +104,12 @@ let dbInsertMany = (collection: Collection, docs: object[]) => {
   });
 }
 
-let dbUpdateOne = (collection: Collection, query: string | object, updateQuery: object) => {
+async function dbUpdateOne(nd: nedb, query: string | object, updateQuery: object): Promise<any> {
   // https://github.com/louischatriot/nedb#updating-documents
   let nedbUpdateQuery = { $set: updateQuery }
   query = wrapIdToQuery(query);
   return new Promise<any>((resolve, reject) => {
-    db.get(collection).update(
+    nd.update(
       query,
       nedbUpdateQuery,
       { returnUpdatedDocs: true },
@@ -114,7 +124,7 @@ let dbUpdateOne = (collection: Collection, query: string | object, updateQuery:
   });
 }
 
-let dbUpdateMany = (collection: Collection, query: string[] | object, updateQuery: object) => {
+async function dbUpdateMany(nd: nedb, query: string[] | object, updateQuery: object): Promise<any[]> {
   // https://github.com/louischatriot/nedb#updating-documents
   if(isEmptyArray(query)) {
     return Promise.resolve([]);
@@ -122,7 +132,7 @@ let dbUpdateMany = (collection: Collection, query: string[] | object, updateQuer
   let nedbUpdateQuery = { $set: updateQuery };
   query = wrapIdsToQuery(query);
   return new Promise<any[]>((resolve, reject) => {
-    db.get(collection).update(
+    nd.update(
       query,
       nedbUpdateQuery,
       { returnUpdatedDocs: true, multi: true },
@@ -137,10 +147,10 @@ let dbUpdateMany = (collection: Collection, query: string[] | object, updateQuer
   });
 }
 
-let dbFindOne = (collection: Collection, query: string | object) => {
+async function dbFindOne(nd: nedb, query: string | object): Promise<any> {
   query = wrapIdToQuery(query);
   return new Promise<any | null>((resolve, reject) => {
-    db.get(collection).findOne(query, (err, doc) => {
+    nd.findOne(query, (err, doc) => {
       if(err) {
         reject(err);
       } else {
@@ -150,13 +160,13 @@ let dbFindOne = (collection: Collection, query: string | object) => {
   });
 }
 
-let dbFindMany = (collection: Collection, query: string[] | object, sortQuery: object = {}) => {
+async function dbFindMany(nd: nedb, query: string[] | object, sortQuery: object = {}): Promise<any[]> {
   if(isEmptyArray(query)) {
     return Promise.resolve([]);
   }
   query = wrapIdsToQuery(query);
   return new Promise<any[]>((resolve, reject) => {
-    db.get(collection).find(query).sort(sortQuery).exec((err, docs: any[]) => {
+    nd.find(query).sort(sortQuery).exec((err, docs: any[]) => {
       if(err) {
         reject(err);
       } else {
@@ -166,10 +176,10 @@ let dbFindMany = (collection: Collection, query: string[] | object, sortQuery: o
   });
 }
 
-let dbRemoveOne = (collection: Collection, query: string | object) => {
+async function dbRemoveOne(nd: nedb, query: string | object): Promise<boolean> {
   query = wrapIdToQuery(query);
   return new Promise<boolean>((resolve, reject) => {
-    db.get(collection).remove(query, { /* options */ }, (err, numRemoved) => {
+    nd.remove(query, { /* options */ }, (err, numRemoved) => {
       if(err) {
         reject(err);
       } else {
@@ -183,13 +193,13 @@ let dbRemoveOne = (collection: Collection, query: string | object) => {
   });
 }
 
-let dbRemoveMany = (collection: Collection, query: string[] | object) => {
+async function dbRemoveMany(nd: nedb, query: string[] | object): Promise<number> {
   if(isEmptyArray(query)) {
-    return Promise.resolve([]);
+    return Promise.resolve(0);
   }
   query = wrapIdsToQuery(query);
   return new Promise<number>((resolve, reject) => {
-    db.get(collection).remove(query, { multi: true }, (err, numRemoved) => {
+    nd.remove(query, { multi: true }, (err, numRemoved) => {
       if(err) {
         reject(err);
       } else {
diff --git a/server/src/utils/segments.ts b/server/src/utils/segments.ts
index 01b9c00..b94aec3 100644
--- a/server/src/utils/segments.ts
+++ b/server/src/utils/segments.ts
@@ -113,7 +113,7 @@ export class IntegerSegmentsSet {
         }
         return s;
       }, null);
-      push(_.last(this._segments).to + 1, Infinity);
+      push(this._segments[this._segments.length - 1].to + 1, Infinity);
     }
     return new IntegerSegmentsSet(invertedSegments, true);
   }
diff --git a/server/tsconfig.json b/server/tsconfig.json
index d5cef27..291b73f 100644
--- a/server/tsconfig.json
+++ b/server/tsconfig.json
@@ -2,6 +2,7 @@
   "compilerOptions": {
     "sourceMap": true,
     "module": "commonjs",
-    "target": "es6"
+    "target": "es6",
+    // "strict": true
   }
 }