diff --git a/src/main/java/org/hbp/mip/data/Database.java b/src/main/java/org/hbp/mip/data/Database.java
index 89794187327ed39e2072a23933aa34118eb810d6..7d2cbf10056cc5fff1117cb5582820d36c835430 100644
--- a/src/main/java/org/hbp/mip/data/Database.java
+++ b/src/main/java/org/hbp/mip/data/Database.java
@@ -32,7 +32,7 @@ public class Database {
             e.printStackTrace();
         }
 
-        // Parse JSON
+        // Deserialize JSON
         Gson gson = new Gson();
         Group rootGroup = gson.fromJson(data, Group.class);
 
@@ -52,9 +52,9 @@ public class Database {
             e.printStackTrace();
         }
 
-        // Parse JSON
+        // Deserialize JSON
         Gson gson = new Gson();
-        Type listVariablesType = new TypeToken<LinkedList<Variable>>(){}.getType();
+        Type listVariablesType = new TypeToken<List<Variable>>(){}.getType();
         List<Variable> variables = gson.fromJson(data, listVariablesType);
 
         // Sync groups and values with DB
diff --git a/src/main/java/org/hbp/mip/model/Article.java b/src/main/java/org/hbp/mip/model/Article.java
index 1556239706bc07240be0e834675ae2f17505d1aa..20b2ff706b12e2019e000c5e6dced1b0afb13071 100644
--- a/src/main/java/org/hbp/mip/model/Article.java
+++ b/src/main/java/org/hbp/mip/model/Article.java
@@ -6,6 +6,7 @@ package org.hbp.mip.model;
 
 import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
 import com.fasterxml.jackson.annotation.JsonProperty;
+import com.fasterxml.jackson.databind.annotation.JsonSerialize;
 import io.swagger.annotations.ApiModel;
 import io.swagger.annotations.ApiModelProperty;
 
@@ -19,6 +20,7 @@ import java.util.List;
 @ApiModel(description = "")
 @javax.annotation.Generated(value = "class io.swagger.codegen.languages.SpringMVCServerCodegen", date = "2016-01-06T09:32:22.266Z")
 @JsonIgnoreProperties(value = { "id" })
+@JsonSerialize(include = JsonSerialize.Inclusion.NON_NULL)
 public class Article {
     @Id
     @GeneratedValue(strategy = GenerationType.IDENTITY)
diff --git a/src/main/java/org/hbp/mip/model/Chart.java b/src/main/java/org/hbp/mip/model/Chart.java
index 153ed8b54aa6e1b13a82603f32f7676271f40c0a..3732c255ed6df0d3255d06e16c6b2c005bf4fb10 100644
--- a/src/main/java/org/hbp/mip/model/Chart.java
+++ b/src/main/java/org/hbp/mip/model/Chart.java
@@ -6,6 +6,7 @@ package org.hbp.mip.model;
 
 import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
 import com.fasterxml.jackson.annotation.JsonProperty;
+import com.fasterxml.jackson.databind.annotation.JsonSerialize;
 import io.swagger.annotations.ApiModel;
 import io.swagger.annotations.ApiModelProperty;
 
@@ -18,6 +19,7 @@ import java.util.List;
 @ApiModel(description = "")
 @javax.annotation.Generated(value = "class io.swagger.codegen.languages.SpringMVCServerCodegen", date = "2016-01-06T09:32:22.266Z")
 @JsonIgnoreProperties(value = { "id" })
+@JsonSerialize(include = JsonSerialize.Inclusion.NON_NULL)
 public class Chart {
     @Id
     @GeneratedValue(strategy = GenerationType.IDENTITY)
diff --git a/src/main/java/org/hbp/mip/model/ChartConfigSet.java b/src/main/java/org/hbp/mip/model/ChartConfigSet.java
index 9e54dbf64d50d90e8337257301d09620b421831a..f98836d492d941b24d0ae64c116d9bc23fd2e471 100644
--- a/src/main/java/org/hbp/mip/model/ChartConfigSet.java
+++ b/src/main/java/org/hbp/mip/model/ChartConfigSet.java
@@ -6,6 +6,7 @@ package org.hbp.mip.model;
 
 import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
 import com.fasterxml.jackson.annotation.JsonProperty;
+import com.fasterxml.jackson.databind.annotation.JsonSerialize;
 import io.swagger.annotations.ApiModel;
 import io.swagger.annotations.ApiModelProperty;
 
@@ -16,6 +17,7 @@ import javax.persistence.*;
 @ApiModel(description = "")
 @javax.annotation.Generated(value = "class io.swagger.codegen.languages.SpringMVCServerCodegen", date = "2016-01-06T09:32:22.266Z")
 @JsonIgnoreProperties(value = { "id" })
+@JsonSerialize(include = JsonSerialize.Inclusion.NON_NULL)
 public class ChartConfigSet {
     @Id
     @GeneratedValue(strategy = GenerationType.IDENTITY)
diff --git a/src/main/java/org/hbp/mip/model/Dataset.java b/src/main/java/org/hbp/mip/model/Dataset.java
index e023ba949093a12ecfac44922c51b9f14085558f..3a4591d1917c8ce6c2a956ef28923297f8cf4df7 100644
--- a/src/main/java/org/hbp/mip/model/Dataset.java
+++ b/src/main/java/org/hbp/mip/model/Dataset.java
@@ -6,6 +6,7 @@ package org.hbp.mip.model;
 
 import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
 import com.fasterxml.jackson.annotation.JsonProperty;
+import com.fasterxml.jackson.databind.annotation.JsonSerialize;
 import io.swagger.annotations.ApiModel;
 import io.swagger.annotations.ApiModelProperty;
 
@@ -19,6 +20,7 @@ import java.util.List;
 @ApiModel(description = "")
 @javax.annotation.Generated(value = "class io.swagger.codegen.languages.SpringMVCServerCodegen", date = "2016-01-06T09:32:22.266Z")
 @JsonIgnoreProperties(value = { "id" })
+@JsonSerialize(include = JsonSerialize.Inclusion.NON_NULL)
 public class Dataset {
     @Id
     @GeneratedValue(strategy = GenerationType.IDENTITY)
diff --git a/src/main/java/org/hbp/mip/model/Filter.java b/src/main/java/org/hbp/mip/model/Filter.java
index 7d2ccc430754aaddf4a2ce18890d7b1183f29854..2a0068cf7ba67a93323510de9d534a9a2a45838a 100644
--- a/src/main/java/org/hbp/mip/model/Filter.java
+++ b/src/main/java/org/hbp/mip/model/Filter.java
@@ -6,6 +6,7 @@ package org.hbp.mip.model;
 
 import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
 import com.fasterxml.jackson.annotation.JsonProperty;
+import com.fasterxml.jackson.databind.annotation.JsonSerialize;
 import io.swagger.annotations.ApiModel;
 import io.swagger.annotations.ApiModelProperty;
 
@@ -16,6 +17,7 @@ import javax.persistence.*;
 @ApiModel(description = "")
 @javax.annotation.Generated(value = "class io.swagger.codegen.languages.SpringMVCServerCodegen", date = "2016-01-06T09:32:22.266Z")
 @JsonIgnoreProperties(value = { "id" })
+@JsonSerialize(include = JsonSerialize.Inclusion.NON_NULL)
 public class Filter {
     @Id
     @GeneratedValue(strategy = GenerationType.IDENTITY)
diff --git a/src/main/java/org/hbp/mip/model/Group.java b/src/main/java/org/hbp/mip/model/Group.java
index 4279b520d687cc8714a5cb75eae572809d46226d..25a783c48f3a24ac3d039b40432a68425972b038 100644
--- a/src/main/java/org/hbp/mip/model/Group.java
+++ b/src/main/java/org/hbp/mip/model/Group.java
@@ -6,6 +6,7 @@ package org.hbp.mip.model;
 
 import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
 import com.fasterxml.jackson.annotation.JsonProperty;
+import com.fasterxml.jackson.databind.annotation.JsonSerialize;
 import io.swagger.annotations.ApiModel;
 import io.swagger.annotations.ApiModelProperty;
 
@@ -18,6 +19,7 @@ import java.util.List;
 @ApiModel(description = "")
 @javax.annotation.Generated(value = "class io.swagger.codegen.languages.SpringMVCServerCodegen", date = "2016-01-06T09:32:22.266Z")
 @JsonIgnoreProperties(value = { "id" })
+@JsonSerialize(include = JsonSerialize.Inclusion.NON_NULL)
 public class Group {
     @Id
     @GeneratedValue(strategy = GenerationType.IDENTITY)
diff --git a/src/main/java/org/hbp/mip/model/Model.java b/src/main/java/org/hbp/mip/model/Model.java
index c3e96b7a4d7d832c07bc09683df5f2d1104bf056..bec3eb9df9f277a469e2e665ddfc3b609833154a 100644
--- a/src/main/java/org/hbp/mip/model/Model.java
+++ b/src/main/java/org/hbp/mip/model/Model.java
@@ -6,6 +6,7 @@ package org.hbp.mip.model;
 
 import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
 import com.fasterxml.jackson.annotation.JsonProperty;
+import com.fasterxml.jackson.databind.annotation.JsonSerialize;
 import io.swagger.annotations.ApiModel;
 import io.swagger.annotations.ApiModelProperty;
 
@@ -17,6 +18,7 @@ import java.util.Date;
 @ApiModel(description = "")
 @javax.annotation.Generated(value = "class io.swagger.codegen.languages.SpringMVCServerCodegen", date = "2016-01-06T09:32:22.266Z")
 @JsonIgnoreProperties(value = { "id" })
+@JsonSerialize(include = JsonSerialize.Inclusion.NON_NULL)
 public class Model {
     @Id
     @GeneratedValue(strategy = GenerationType.IDENTITY)
diff --git a/src/main/java/org/hbp/mip/model/Query.java b/src/main/java/org/hbp/mip/model/Query.java
index ea4840dd9268b13fb92cc500b0e8ee5006fe25c4..bc29b8c576fc6b59aae24a6acc66e8379255a372 100644
--- a/src/main/java/org/hbp/mip/model/Query.java
+++ b/src/main/java/org/hbp/mip/model/Query.java
@@ -6,6 +6,7 @@ package org.hbp.mip.model;
 
 import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
 import com.fasterxml.jackson.annotation.JsonProperty;
+import com.fasterxml.jackson.databind.annotation.JsonSerialize;
 import io.swagger.annotations.ApiModel;
 import io.swagger.annotations.ApiModelProperty;
 
@@ -18,6 +19,7 @@ import java.util.List;
 @ApiModel(description = "")
 @javax.annotation.Generated(value = "class io.swagger.codegen.languages.SpringMVCServerCodegen", date = "2016-01-06T09:32:22.266Z")
 @JsonIgnoreProperties(value = { "id" })
+@JsonSerialize(include = JsonSerialize.Inclusion.NON_NULL)
 public class Query {
     @Id
     @GeneratedValue(strategy = GenerationType.IDENTITY)
diff --git a/src/main/java/org/hbp/mip/model/Tag.java b/src/main/java/org/hbp/mip/model/Tag.java
index 5a76c379575a5820a9d124f8c6c3d80c504221e1..14adacc214b500c3f6b8de02abcbee39acc54fcf 100644
--- a/src/main/java/org/hbp/mip/model/Tag.java
+++ b/src/main/java/org/hbp/mip/model/Tag.java
@@ -6,6 +6,7 @@ package org.hbp.mip.model;
 
 import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
 import com.fasterxml.jackson.annotation.JsonProperty;
+import com.fasterxml.jackson.databind.annotation.JsonSerialize;
 import io.swagger.annotations.ApiModel;
 import io.swagger.annotations.ApiModelProperty;
 
@@ -16,6 +17,7 @@ import javax.persistence.*;
 @ApiModel(description = "")
 @javax.annotation.Generated(value = "class io.swagger.codegen.languages.SpringMVCServerCodegen", date = "2016-01-06T09:32:22.266Z")
 @JsonIgnoreProperties(value = { "id" })
+@JsonSerialize(include = JsonSerialize.Inclusion.NON_NULL)
 public class Tag {
     @Id
     @GeneratedValue(strategy = GenerationType.IDENTITY)
diff --git a/src/main/java/org/hbp/mip/model/User.java b/src/main/java/org/hbp/mip/model/User.java
index d18a876742b48a24143090686f0ba8774f8d34d4..e2023cc27de8eb6d235c92a3cc6cb6cea0af35e1 100644
--- a/src/main/java/org/hbp/mip/model/User.java
+++ b/src/main/java/org/hbp/mip/model/User.java
@@ -5,6 +5,7 @@
 package org.hbp.mip.model;
 
 import com.fasterxml.jackson.annotation.JsonProperty;
+import com.fasterxml.jackson.databind.annotation.JsonSerialize;
 import io.swagger.annotations.ApiModel;
 import io.swagger.annotations.ApiModelProperty;
 
@@ -18,6 +19,7 @@ import java.util.regex.Pattern;
 @Table(name = "`user`")
 @ApiModel(description = "")
 @javax.annotation.Generated(value = "class io.swagger.codegen.languages.SpringMVCServerCodegen", date = "2016-01-06T09:32:22.266Z")
+@JsonSerialize(include = JsonSerialize.Inclusion.NON_NULL)
 public class User {
     @Id
     @GeneratedValue(strategy = GenerationType.IDENTITY)
diff --git a/src/main/java/org/hbp/mip/model/Value.java b/src/main/java/org/hbp/mip/model/Value.java
index d619c72ce7f3304513fcc927744bed3fdceede74..cbca0c7d90da7b29ffaa248bfd9031f3a6db6d2f 100644
--- a/src/main/java/org/hbp/mip/model/Value.java
+++ b/src/main/java/org/hbp/mip/model/Value.java
@@ -6,6 +6,7 @@ package org.hbp.mip.model;
 
 import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
 import com.fasterxml.jackson.annotation.JsonProperty;
+import com.fasterxml.jackson.databind.annotation.JsonSerialize;
 import io.swagger.annotations.ApiModel;
 import io.swagger.annotations.ApiModelProperty;
 
@@ -16,6 +17,7 @@ import javax.persistence.*;
 @ApiModel(description = "")
 @javax.annotation.Generated(value = "class io.swagger.codegen.languages.SpringMVCServerCodegen", date = "2016-01-06T09:32:22.266Z")
 @JsonIgnoreProperties(value = { "id" })
+@JsonSerialize(include = JsonSerialize.Inclusion.NON_NULL)
 public class Value {
     @Id
     @GeneratedValue(strategy = GenerationType.IDENTITY)
diff --git a/src/main/java/org/hbp/mip/model/Variable.java b/src/main/java/org/hbp/mip/model/Variable.java
index 044cca2f772cfa00e519a4cfdd7156cf005b0f70..77c5e8d3356f643bd0f774118b6cc5226b57be66 100644
--- a/src/main/java/org/hbp/mip/model/Variable.java
+++ b/src/main/java/org/hbp/mip/model/Variable.java
@@ -6,6 +6,7 @@ package org.hbp.mip.model;
 
 import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
 import com.fasterxml.jackson.annotation.JsonProperty;
+import com.fasterxml.jackson.databind.annotation.JsonSerialize;
 import io.swagger.annotations.ApiModel;
 import io.swagger.annotations.ApiModelProperty;
 
@@ -18,21 +19,25 @@ import java.util.List;
 @ApiModel(description = "")
 @javax.annotation.Generated(value = "class io.swagger.codegen.languages.SpringMVCServerCodegen", date = "2016-01-06T09:32:22.266Z")
 @JsonIgnoreProperties(value = { "id" })
+@JsonSerialize(include = JsonSerialize.Inclusion.NON_NULL)
 public class Variable {
     @Id
     @GeneratedValue(strategy = GenerationType.IDENTITY)
     private Long id = null;
-    @ManyToOne(fetch = FetchType.EAGER)
-    private Group group = null;
     @Column(unique = true)
     private String code = null;
     private String label = null;
     private String type = null;
     private Integer length = null;
+    private Double minValue = null;
+    private Double maxValue = null;
+    private String units = null;
     private Boolean isVariable = null;
     private Boolean isGrouping = null;
     private Boolean isCovariable = null;
     private Boolean isFilter = null;
+    @ManyToOne(fetch = FetchType.EAGER)
+    private Group group = null;
     @ManyToMany(fetch = FetchType.EAGER)
     private List<Value> values = new LinkedList<Value>();
 
@@ -181,6 +186,45 @@ public class Variable {
         this.values = values;
     }
 
+    /**
+     * Minimum value (only for numbers)
+     **/
+    @ApiModelProperty(value = "Minimum value (only for numbers)")
+    @JsonProperty("minValue")
+    public Double getMinValue() {
+        return minValue;
+    }
+
+    public void setMinValue(Double minValue) {
+        this.minValue = minValue;
+    }
+
+    /**
+     * Maximum value (only for numbers)
+     **/
+    @ApiModelProperty(value = "Maximum value (only for numbers)")
+    @JsonProperty("maxValue")
+    public Double getMaxValue() {
+        return maxValue;
+    }
+
+    public void setMaxValue(Double maxValue) {
+        this.maxValue = maxValue;
+    }
+
+    /**
+     * Units
+     **/
+    @ApiModelProperty(value = "Units")
+    @JsonProperty("units")
+    public String getUnits() {
+        return units;
+    }
+
+    public void setUnits(String units) {
+        this.units = units;
+    }
+
     @Override
     public String toString() {
         StringBuilder sb = new StringBuilder();